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BRAKE saxon V6

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Post by westton Tue Jun 08, 2021 3:11 am

Good morning all
On my Saxon V6, I have brake problems. It brakes very poorly, you have to press the pedal very very hard to obtain fairly weak braking. Practically impossible to block the wheels even at 20 km / h. So not very reassuring.
I put on a new Mustang 66 servo and master cylinder, I put new double piston front calipers, new linings at the rear, new brake hoses at the front. It does not brake better ...
The only thing that I did not change is the brass part on which the 2 master cylinder tubes arrive and leave by a single tube on the rear drums and by 2 tubes at the front calipers.
Does this part have a role on the pressure limitation? or on the distribution of power between the front and the rear. What it do ?
I have never seen this type of part on European cars ...
I ordered a new one in the USA, I am waiting for it.
Other than that I don't see what can cause it to not brake well?
I even put soft racing pads!
Obviously done dozens of purges, new liquid ....
Do you have an idea ?
Thanks

westton

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Post by Hotrod Tue Jun 08, 2021 7:01 pm

It's very hard to diagnose brake issues on a custom car without knowing the exact parts involved.  The brake system seems very simple, but it relies on a complex interaction of parts to work correctly.

There are many things that could cause the problems you describe.  I'll just give a list.

Master cylinder bore size versus caliper and wheel cylinder bore.  Your 66 Mustang master cylinder statement is confusing since it is a single chamber master.  You seem to say later that it has 2 lines that go to a distribution block, that would imply a dual circuit MC.  The 76 Mustang, however, does have a dual circuit master.  Both the 66 and 76 MC's have a .94 bore according to the info I have available.  With a power booster (servo), that should be OK if it's working as it should.

The servo needs good vacuum to work.  You will usually need at least 15 inches of vacuum or more to get one to actually provide usable boost.  More vacuum is always better in this case.  Often, hot cams will not make a lot of vacuum.  It's just how they are.  Ignition timing can affect vacuum as well.  Late timing will lower vacuum.  Also, vacuum leaks will hurt servo performance.  This could include manifold leaks at the gaskets.  On a V8 or V6, this can include leaks under the intake in the lifter valley.  A basically stock engine should produce somewhere around 18-20 inches of vacuum.  A hot cam might lower this to 10 inches or less.  The only way to diagnose this is to connect a vacuum gauge to the manifold and see what you have.

The servo may be bad even though it is new.  Many parts are sold that are defective.

Caliper bore size is important too.  You mention 2 piston calipers. The Saxon used Chevette front ends, I believe.  They may have used Mustang II front ends on some.  Both of those cars used single bore  calipers, so that would indicate that you have some other brakes.  If they are aftermarket, you should be able to contact the manufacturer and see what they recommend for a MC bore size.

They brass block you asked about is distribution block.  They can be made several ways.  They may be just a complicated "T" fitting or they could have internal controls for the brake fluid.  Some have a device called a hold off valve in the front circuit.  This restricts front brake activation until around 100 PSI is reached on the rear circuit.  This allows the rear drum brakes to catch up to the front discs.  The drums have farther to travel before they actually start working.  the block could also contain a proportioning valve in the rear circuit. This will limit rear brake line pressure to help prevent rear lock up.  This is because drum brakes work at lower line pressure that discs.  Rear lock up will cause a car to spin on slick roads or in a curve.  the block will usually contain a floating valve that is balanced between the front and rear line pressure.  If a leak occurs on the front, this valve will shift to the front and shut off that circuit.  it will work the same way if a leak occurs on the rear line and seal that line.  This helps limit fluid loss and helps the dual circuit MC maintain braking on one end of the car if you blow out a line or hose on the other end.

One thing that can happen is the shuttle valve I described can move one way or another while bleeding the brakes.  This will shut off whatever end had the lowest pressure.  It is not self centering.  You have to open a bleeder on the open side to shift the valve back to the center.  If one end of the car is sealed off by the valve you could have a very firm pedal, but virtually no brakes on one end of the car.  This would essentially leave you with half of your brakes, causing the problem you describe.

The final area that comes to mind is pedal ratio.  You need somewhere around 4 - 6 to 1 leverage ratio on the brake pedal.  Higher is better.  This would have been designed in on the original factory pedal setup, no matter what car the pedals came from.  If the pedal was shortened to fit in the Saxon, then you probably don't have enough force multiplication in the pedal.

Sorry I can't be more specific to your problem.  Without having the car in front of me, it's just a broad guess as to the issue. There could be other issues, this list is just the ones that came to mind.
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Post by westton Wed Jun 09, 2021 4:27 am

Thanks for your help.
It is indeed necessary to specify 2 or 3 points. It's a car I've had for about a year that would total 1,500 miles ... it's a factory Saxon CR. Registered in February 86.
The crankset has apparently not been modified, no traces or signs of cuts and welding. It only has the clutch and the brakes, the accelerator is independent
The running gears are indeed Chevettes. Front and rear.
This car has never braked since I got it!
Everything, absolutely everything seemed new without the slightest wear which could confirm the very low mileage., Discs, pads, drums .... new !!
Impossible to block the front wheels !!! very, very hard the back.
The servo should work because when the engine is running and when the engine has stopped, the change in the pedal is felt. I'm going to test for depression but in my opinion it's OK.
The master cylinder is double separate circuits, so it should not be a mustang 66 model ....
The 2 tubes connecting the master cylinder and the brass case are of different diameter. I had never seen this on a European car ... 3/16 and 1/4 ???? curious, but it is perhaps in connection with this brass case. Unknown in Europe .... In Europe the front brakes are always dissociated from the rear brakes. On basic cars there was nothing on the rear circuit to limit the pressure, it is indeed the larger volume of liquid to be moved which limited the pressure. On slightly more advanced models, there was either a mechanical pressure limiter, set at the factory, or a valve system connected to the rear suspension and limiting according to the inclination during braking.
I replaced the front calipers which were single pistons (chevette) with double pistons from the opel Kadett GTE, the very sporty model of the Kadett (redesigned chevette and made in Germany. I thought the lack of braking manifested front was linked to the calipers either undersized or damaged internally (car which is 35 years old and never driven).
I should point out that the car does not dive in the front when braking ... which is not normal.
But even with these calipers supposed to be more powerful, nothing better, so racing pads to try .... always the same.
I tested on candles, I have a little over 40 bar of pressure (almost 600 PSI if I am not mistaken) in the circuit at each wheel! Tested one by one, I cannot do all 4 simultaneously, this are correct a priori, maybe a little tight on the front but nevertheless in principle sufficient.
I wanted to test on the brass block but obviously the threads do not correspond to any of my adapters in the kit that I was loaned.
I don't understand .... with 40 bars on each wheel it should brake. Unless by rolling this brass block "neutralizes" the front circuit ....
On jack stands, braking, even with a very large lever, it is impossible to turn the front wheels.
So that's why I suspect this brass block to act and not to be a simple T serving as a support ... Once again, it's unknown in Europe. So nobody knows what this is for " thing ".
Yet I know people who are in the world of car racing, they do not know!
There is no adjustment screw on it, nothing that seems to allow access to an interior mechanism. Not seen a reference.
How do we recognize what action he is doing? Is there a method to test this piece?
I'm expecting a new one, but I ordered it from the USA just with the product photo ... so no idea how it works.
There is more than that of origin ... I changed everything!

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Post by westton Wed Jun 09, 2021 4:49 am

I will try your idea of a floating valve to be purged differently than usual (in Europe). With us we always start with the furthest wheel.
On the other hand, I did not understand how to balance the position of the valve while purging.
Could you explain to me, detailing more, because I only speak English moderately .....
Merci d'avance!

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Post by Hotrod Wed Jun 09, 2021 11:03 am

The distribution block can have several names.  Some call it a combination valve and some call it a proportioning valve.

There is usually a switch in the center of the block that connects to a light on the dash of US cars.  This light comes on when the shuttle valve in the block shifts to one end or the other.  I have recentered them by watching this light and bleeding one end of the car or the other to get the shuttle valve to shift back to the center.  There is a tool available to lock the shuttle valve in the center while bleeding the brakes.  It is a very simple too and could easily be made if you have access to a metal lathe.

Here are some YouTube videos that describe these valves better than I can.

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You may even be able to set up the closed captioning to do a French translation.  I tried that on one of the videos and it seemed to translate ok, but I only understand a très petite amount of French and that is Cajun from south Louisiana.  These videos talk about muscle cars and trucks, but all the combination valves work the same, so they do apply to your car.

The 2 different line sizes is common on north american cars.  Don't worry about that.

Yes, if the shuttle valve is not centered and favoring the front circuit, it can completely seal off the front brakes when you apply the pedal.


Last edited by Hotrod on Wed Jun 09, 2021 11:17 am; edited 1 time in total
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Post by Hotrod Wed Jun 09, 2021 11:14 am

Oh I should add that most disc brake systems generally require 800 to 1200 PSI (40 to 80 BAR) to function correctly.  Since you say that you have 40 BAR, your system may need much higher pressure.  I know the old Jaguar systems were well over a 1000 PSI, approximately 70+ BAR.

I still think that you might have a brake pedal leverage problem.  Check out these websites:

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Once you have calculated the brake pedal ratio that you have now, you should be able to calculate the line pressure from the MC bore size.
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Post by David V. Wed Jun 09, 2021 3:33 pm

Hotrod a raison, je pense que la raison la plus probable pour le manque de freinage c'est que la valve dans le bloc en laiton qu'on appelle une "proportioning valve" peut bloquer les lignes de friens avant si on n'effectue pas le saignage des freins correctement. Il existe un petit outil en plastique qu'on peut visser dans le bloc pour pouvoir saigner les friens sans que la valve se deplace. En temps normal cette valve permet aux freins arriere de commencer le freinage avant les friens avant pour ne pas destabiliser la voiture lors du freinage. Bonne chance! Smile Desole pour mon accent Quebecois.

For my english speaking friends: I think Hotrod is right, the most likely cause for the lack of front brakes is the valve in the proportioning valve that can block off the front lines if the bleeding process isn't done correctly (it's happened to me). There exists a small plastic tool that you can screw into the top of the proportioning valve while bleeding the brakes to prevent this from happening. Under normal circumstances the purpose of the valve is to allow the rear brakes to engage slightly before the front brakes to not destabilize the car under braking. Good luck! Smile
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Post by westton Wed Jun 09, 2021 6:21 pm

thank you very much, this gives me some ideas to try.
I am going to look at this valve, I am waiting for the new one which should arrive soon.
I keep you posted on my progress on the problem!
Thank you again, because here it is not to use at all and even by going to French forums dedicated to the Mustang, they completely ignore the function of this valve. They seem that for them it only serves as a pressure loss switch.
Merci cousin Québécois Laughing Laughing

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Post by Hotrod Wed Jun 09, 2021 11:16 pm

OK, I have a little time to get back on this.  

I did some checking and found out that the Opel either had 44 or 48 mm bore calipers, most likely.  On a caliper with opposed pistons, you only use the area of one side.  In other words, the 2 pistons don't give you twice the braking power.  The Chevette calipers were probably 42 mm, so the Opel parts would be an upgrade.  No problems there if they are working as they should.

The Opel would have had a master cylinder bore of somewhere around 20 mm (.787 inches).  The Chevette master was larger at .875 inches.  A Mustang II master is even larger at .937 inches.

Either of the larger bore master cylinders will decrease brake pressure from what the Opel Kadett originally had.  That will increase the amount of foot pressure on the pedal.  A MC with a bore of .75 inches would lower your required pedal effort.

Just something else to look at if nothing else works.
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Post by westton Fri Jun 11, 2021 4:06 pm

I received the valve, but obviously no compatible thread .... I modify my tubes, to test. I have to redo the fittings.
Small question, it seems curious to me, the master cylinder piston on the flywheel side powers the front brakes. The master cylinder piston on the car's radiator side powers the rear brakes.
It seems curious to me ... Is it like that on your cars?
It seemed to me that the first piston attacks the rear brakes and the second the front calipers normally ....

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Post by David V. Fri Jun 11, 2021 4:34 pm

Oui c'est comme ca sur ma voiture, j'ai aussi trouve ca etrange quand je l'ai remplace il y a deux ans. Et pourtant mon systeme de freinage est de marque Chevrolet au lieu de Ford.

"Yes it's like that on my car as well, even though my brake system is from a chevy, found it strange as well when I replaced it 2 years ago.

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Post by Hotrod Fri Jun 11, 2021 4:40 pm

As a general rule, the rear port (flywheel side) has a slightly more capacity to move fluid than the front port.  This is due to the way the compensating ports in the master cylinder are laid out.  Since the disc calipers have much larger pistons than the rear drum brake cylinders, the front brakes are plumbed to this port.

However, if your MC has one fluid bowl larger than the other, then the larger bowl end is plumbed to the front brakes.  The larger bowl size holds more fluid to make up for front brake pad wear in the larger piston disc calipers.

The hold off valve in the brass block prevents the front brakes from operating until the preset pressure is reached.


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Post by westton Fri Jun 11, 2021 4:45 pm

yes there is a large jar and a small one. The large one is on the driver's side, on the front calipers

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Post by westton Sat Jun 12, 2021 10:19 am

Well, after having made the fittings, I reassembled everything, purged with a DIY tool to block the valve, no progress!
Always the same thing, pedal soft with weak braking, then becomes very hard suddenly. By forcing the pedal, only the rear wheels block very lightly.
Various tests show me that the brakes brake at the start of the pedal stroke (we feel the car plunged on the front slightly), no progress of the braking (the pedal goes down easily), then the pedal goes down only with great difficulty. . When pressing really hard, the rear wheels lock very little.
I have the impression that the brakes work on the first third of the pedal stroke, then on almost the next 2 thirds nothing happens until the end of the stroke. And on the last centimeter we send a supplement. pressure only on the rear. The pedal refuses to go down any further. If I open a bleeder, the pedal goes down, it means that the pressure is enormous to prevent me from lowering the pedal, but that this pressure does not reach the wheels ...
I don't understand what is going on!
I am going to order a connection T and I am going to make a very simple circuit by removing this valve .... A circuit for the front and one for the rear completely independent, European style [You must be registered and logged in to see this image.] !

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Post by westton Sat Jun 12, 2021 10:32 am

With this simple circuit, I hope to fix the problem or at least figure out what is wrong ....
If with 2 really independent circuits I still don't get the front wheels blocked, I will probably have to try with another master cylinder ..

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Post by Hotrod Sat Jun 12, 2021 10:57 am

As I said in the beginning, it is very difficult to diagnose a problem from long distance.

The actions you describe sounds like there is still air in the front brakes and the shuttle valve is moving to close of the front brakes when you press the pedal.  This would explain why the pedal is soft initially and then gets very hard.  When the pressure gets high enough, the proportioning valve is bottoming out and sending pressure to the rear brakes.  This would explain why you get some braking on the rear at the end.

Removing the valve might help, but it shouldn't.  If the system is working as it should, the valve doesn't affect anything.  Millions of cars in the US have them with no issues.  It is possible that the caliper swap you did is requiring more fluid in the front brakes and that is tricking the valve into reacting as if there is a leak in the front brakes.

I am still of the belief that you have a pressure issue.   Even if you get the system to work on both ends of the car, I think you will have a very hard pedal and little braking.
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Post by westton Sat Jun 12, 2021 11:31 am

I hope you did not take my alusion to a simple European solution wrong .... It's just a joke!
To come back to the brakes, I bleed by suction at each bleeder (depression), and in the old way by pumping on the pedal ...
Moreover, when I pump, purging already done, the pedal stroke does not change, if there was air, the pedal stroke would shorten. This is not the case. Even with 100 pedal strokes the stroke does not vary!

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Post by westton Sat Jun 12, 2021 11:41 am

And then 40 bars, with air in the front circuit ???? possible????
On the other hand, I did not specify it, the pressure at the front rises when I press, at the beginning grow up 50 bars , after down to 40 bars and does not move any more. Indeed as it is my son who was in front of the mano, he did not tell me until after.

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Post by Hotrod Mon Jun 14, 2021 10:32 am

I have tried suction bleeding and did not have good luck with it.  My issue has always been air seeping in around the bleeder screw itself.  

Many years ago, I swapped over to pressure bleeding.  With this technique, the master cylinder is fed pressurized fluid through a special cap and a separate pressurized fluid reservoir.  This technique has never failed me and I have used it on some vehicles that were virtually impossible to bleed any other way.  Here is a video that talks about the process.  There are others.

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Many times, custom brake setups will have high points in the plumbing that are very difficult to bleed.  Manufacturers go to great lengths to prevent this, but a home builder doesn't have the engineering resources to ensure this doesn't happen.  Sometimes, a seemingly insignificant tilt in a component or even the rake the car sits at, will cause high points that trap air bubbles.  I have seen custom cars that required raising the front of the car to get the brakes to bleed.

I could not guarantee that 40 bar is not possible with air still in the system.  Air is compressible and even a small air bubble can use up a lot of pedal travel.  Compressing an air bubble to 40 bar is entirely plausible.  By the time you have compressed the bubble, there is no travel left to increase the pressure any farther.

I am not offended by anything you posted.  I wish I could help you, but I am running out of ideas.
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Post by westton Mon Jun 14, 2021 12:06 pm

I'm glad you didn't misinterpret my words!
. Thanks for your help, I know pressure purge systems. I would have to make a cap from the old cover. But it's worth a try ... to the point where I am! and I really don't have any other idea!
For information, the master cylinder was purged before being put in the car, it was indicated in a manual accompanying the master cylinder servos assembly. He was providing a kit for it.

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Post by Hotrod Mon Jun 14, 2021 1:56 pm

Another thing to add.  Make sure the bleeders on the new calipers are the highest point.  This is sometimes an issue with brake swaps.  I have had to remove the caliper and block the pads apart with wood so that the caliper can be rotated to get the bleeder on top.  After bleeding,  remove the wood and reinstall the caliper. It's very easy to create a small dead area in a caliper that will not bleed if the bleeder is not absolutely on top.
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Post by David V. Tue Jun 15, 2021 5:50 pm

That reminds me, when I did a brake pad change on my car a couple of years ago I discovered that the right side caliper had been installed on the left and the left side caliper on the right which didn't help bleeding at all. It's an easy mistake to make with the MustangII calipers, I wouldn't have noticed if I hadn't looked at diagrams while re-installing and then been confused because my caliper flex-lines were too short all of a sudden. Just a thought
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