Problem on braking system

Hello friends across the Atlantic!

Here I have a 1931 FORD A streetrod ( V8 Chevy and transmision TH300) on which I have a problem of braking until the insoluble!
When I activate the brake pedal, the vehicle stops perfectly for about 10 or 15 braking (or more). Hazy at the 16th brake (or more), the brake pedal goes to the bottom, the red light lights up and there’s almost no brake! :eek:
I first replaced the rear wheel cylinders with brake shoes, then the brake calipers seals, the master cylinder https://www.rockauto.com/en/moreinfo...pt=1836&jsn=16) , then the brake proportion valve https://www.summitracing.com/parts/clp-pv-2, then all brake lines (rigid and flexible).
I bleed the brake circuit without any particular difficulty, by putting the bleeder tool on the PV2 valve. Despite the exchange of all these parts the problem still remains (problem of pressure in the circuit)... ! **)
There is still the brake booster to replace, but for me the symptoms are not related to this problem.
Do any of you have any idea ( diameter of rigid pipes, !? ...) I don’t know what to do anymore.:confused:

Thank you very much for your help.

Jef
France

You have the symptoms of a failing master cylinder from my experience

I'd have to agree. If you are not finding fluid on the wheels or floor - and - if the area behind the master in the vacuum booster is dry, they there is pressure leaking/bypassing in the system internally. Are you certain you used the correct fluid DOT #?

BTW, nice to see you again

Thank you for your answers.
There is no trace of liquid, especially behind the master cylinder. It would be really bad luck to have received a master cylinder defectful...but why not!.
I changed the fluid type with "blue racing fluid" which is very heat resistant and DOT4 compatible. What is amazing is that with a completely new system the dysfunction persists. I tested the booster, it works perfectly.
I’m gonna run a new purge and check my connection connections.

Hello guys;
I made a new bleed of the brake lines, now i'm sure there is not air inside the brake system! Unfortunately my problem is same!:rolleyes: I push the pedal absolutely no brake, (alarm light brake on). After 15 to 20 pedal strokes the car brakes almost normally with a stronger force difference in the rear (brake light is off). Back home again the pedal goes to the bottom...!:eek:
I noticed when I was bleeding my brakes (after installing my new master cylinder) that the rear reservoir is pushing or bubbling up fluid when I push the pedal (kick-back of fluid into the reservoir). Its not air bubbles but fluid pushing up into the reservoir and sometimes even coming out of the reservoir. The front doesnt do that ! Is this normal ?:confused:

Thanks again !

Do you have a return spring on your brake pedal, to pull it back from the master cylinder? With your foot off the brake the actuating rod that goes from the brake pedal to the master cylinder should have a bit of free play between the end of the rod and the master cylinder valve plunger, just a millimeter or less is good.

Is your master cylinder under the floor, putting it lower than the caliper/wheel cylinders? If yes, do you have brake pressure residual valves (link - https://www.speedwaymotors.com/Wilwo...xoCJK4QAvD_BwE ) installed, a 2 pound for the discs and a 10 pound for the drums?

Just a couple of thoughts that popped into my head.

Fluid pushing back into the master is normal. Do you have drum brakes on the rear? The reason I ask is that drum brakes move a lot more fluid each stroke of the pedal, the return springs force the fluid to move back. Discs only return a small amount of fluid and it can be gradual so maybe you couldn't see it. Have someone pry a caliper back while you watch inside the master, I'm sure you'll see it - especially if they do it to hard/fast! But more important to me is that verifies the push rod isn't applying pressure to the master and it has fully returned!
Another thought occurred to me when reading your reply.. You said you tested the booster. HOW? Also, did you bench bleed the new master cylinder? And I mean on the bench, probably clamped into a vise and you could observe the fluid / air moving through the cylinders??? Another thought I have is this a whole new car with all new parts? Or is this an old car that you're bringing back? reason I am asking that is there were some through the frame bulkhead fittings that had a differing size and they would actually trap air in the system.

Hello everyone,

To answer to «rspears» the master cylinder is located in a classical height. There is no return spring at the pedal, it returns correctly thanks to the internal spring of the master cylinder. So there are no pressure regulation valves!
In response to "34_40" yes I have already bled the master cylinder to the workbench. Even if I had done it wrong from the start I think it is right now anyway because I did several bleeds and circulate a lot of fluid in the circuit. There’s no air coming out of all four wheels.
While reflecting there is a small noise of "toc' in the area of the booster push rod. I did not check the play of the rod against the master cylinder... need to do! :HMMM:
I checked the booster in 2 ways: Engine stopped 4 pedal strokes to empty the air and then started the engine, the pedal normally goes down by a third. Then I deconnected the booster’s depression hose, I drove the car and even the problem of pressure drop in the circuit (of course with a very hard pedal but it’s normal!)
Perhaps the problem of this car (Streetrod FORD A 1931) is that it’s equipped with parts of different cars (discs + front axles of 1969-72 Chevelle 4,1L- Monte-Carlo, 73-77 A-Body... etc) with a complete front-end axle of a 1968 Corvair chevrole; the rear axle comes from a Chevrolet CHEVELLE (GM N3969277 "10 bolts") with drums, the master cylinder equips the 1973 Ford Mustang, 1975 + Granada . There is a front/rear propotioning valve PV2 discs/drums (on which I fixed the plug to screw to bled).
I also changed all the piping according to the diameter already installed: 4.75mm on the front circuit, 6 mm on the rear circuit until (the rear rubber hose) then again 4.75mm from the center of the rear axle to each rear wheel.
In fact I replaced all the parts of braking identically by new ones with some difficulty because I had to search on the web the provenance of each of these parts! But perhaps there has been an incompatibility between them since the beginning when the car was built!?:confused: ... How to know!?

I always keep looking....... I want to thank you very warmly for your help!;)

Parts mismatch is a common malady.. My next question is do you know the diameter of the master cylinder? 7/8" , 15/16" , 1" , 1 1/16" , you may need a larger bore diameter to simply move more fluid and also the master must be equipped for disc front and drum rear. If you replaced parts in kind, same sizes with same sizes, you may have just continued the mismatch but from Massachusetts USA, that is simply a guess! But with all the checks/inspections.... it sounds to me like the master is just not supplying enough fluid to make pressure..

The new master cylinder I bought (sold for front discs and rear drums) is exactly this one:
https://www.rockauto.com/en/moreinfo...&pt=1836&jsn=9

The diameter of the piston is not indicated by the manufacturer, but if we take another corresponding brand for exactly the same vehicle it is mentioned the different specifications.
https://www.rockauto.com/en/moreinfo...&pt=1836&jsn=8

So i suppose that RAYBESTOS and CENTRIC master cylinders have same specifications !

Type of Front calipers : https://www.rockauto.com/en/moreinfo...1704&jsn=10737
(DELCO MORAINE 499 on my car)

Rear brake cylinders i bought: https://www.rockauto.com/en/moreinfo...k=110300&jsn=8

It remains to be seen if all this material is compatible together!:confused:

Well.. brakes are the most important part of the vehicle.. I don't think you should be supposing anything. What you have doesn't work so you are going no where fast. You need facts. And why buy a master that fits a Ford when all the other parts come from Chevy? Take a few minutes and look at a company like Wilwood, expensive? YES! but they will provide you excellent tech help and you can get many parts there that could help solve your issues.