brakes Intermittent Hard Brake Pedal-1971 (base)stock brakes

[justadude]

After getting my ignition system dialed, I am now onto my brake system. Initially by car would pull to the left and vacuum assist rarely worked. I inspected the brakes and found a sticking caliper on the front right hand side so I decided to refresh the brakes by.

1.replacing front calipers and pads

2.replacing rear wheel cylinders

3.adjusting the rear shoes

4.replacing the vacuum line from intake to brake booster (with proper vacuum hose 12x19mm)

5. I also power bled the system 

6.I tested the Vacuum Booster to make sure it held vacuum and also that there was no sign of fluid leakage from the master cylinder.

 

All seemed good as test drove it to seat the pads and the when I needed a quick stop it gave me a really hard pedal as if the brake booster wasn't working. Before you say it the one way inline check valve is installed correctly with the arrow pointing towards the intake manifold. 

 

So at this point I can't count on my brakes to work in an emergency situation. When the pedal gets hard is I let it out and push back down on it I do get my vacuum boost. I am kinda stumped at this point. I also tested the one way inline valve with my vacuum gauge and it held vacuum when on the booster side and let vacuum through when on the manifold side. 

 

I also have good vacuum on the manifold, about 20"hg at idle.

 

What am I missing here? I am confident that there is no air in the lines because I don't have a spongy pedal that firms up if I pump it. The only 2 things left that I can think of are that the master cylinder is random binding up internally or that the one way check valve is randomly leaking under fluctuating conditions. Tell me what you think I missed. BTW I don't think my car has a proportioning valve or pressure differential valve. It is a 1971 base with stock brakes.

 

[Simeon]

It sounds like you have a sticky internal air valve. A rebuild or good second hand one is needed. 

 

As as the pedal is pressed this opens the air valve which allow atmospheric pressure into the back of the booster. This  overcomes the spring and pushes the actuator rod out, pressing on the master cylinder. This means that when working the pedal feels 'softer' as the actual piston is being pushed away by pressure in advance of the actual pedal pressure. If it feels hard then the assistance isn't working and you are pushing the whole assembly through its full travel directly. 

[justadude]

8 minutes ago, Simeon said:

It sounds like you have a sticky internal air valve. A rebuild or good second hand one is needed. 

 

As as the pedal is pressed this opens the air valve which allow atmospheric pressure into the back of the booster. This  overcomes the spring and pushes the actuator rod out, pressing on the master cylinder. This means that when working the pedal feels 'softer' as the actual piston is being pushed away by pressure in advance of the actual pedal pressure. If it feels hard then the assistance isn't working and you are pushing the whole assembly through its full travel directly. 

 

Thanks for the reply, I stated earlier that it was intermittent however I found a way to make it repeatable.

 

If I get on the brakes quickly I get the vacuum boost effect. If I start to gradually apply the brakes several hundred feet before I stop by the time I need full brake force the pedal is stiff unless I release it slightly and re-depress it.

 

 

 

[Simeon]

Something within the valve assembly is not sliding open when it should. The only way to access that valve is by opening up the booster and that is considered 'a non user serviceable' item. 

 

What you could try (and I am just making this up) is remove the booster from the car, leave it with the vacuum connected and the engine running and direct some carb cleaner into the air intake at the back around the input push rod as you work the mechanism. This would then flow through and maybe clean the valve on the way through. What I would say is that I have no idea what impact that might have on the diaphragm inside or any internal plastic parts. 

 

Like I say, a rebuild or known good replacement might be better. 

[justadude]

2 hours ago, Simeon said:

Something within the valve assembly is not sliding open when it should. The only way to access that valve is by opening up the booster and that is considered 'a non user serviceable' item. 

 

What you could try (and I am just making this up) is remove the booster from the car, leave it with the vacuum connected and the engine running and direct some carb cleaner into the air intake at the back around the input push rod as you work the mechanism. This would then flow through and maybe clean the valve on the way through. What I would say is that I have no idea what impact that might have on the diaphragm inside or any internal plastic parts. 

 

Like I say, a rebuild or known good replacement might be better. 

Simeon, I am lean towards your logic here. The more I think about it the valve inside the master cylinder that opens and closes the Vacuum and atmospheric pressure seems to be not functioning properly. I just want to exhaust the simplest stuff first before I go order a new master. BTW the only master I can find is made by Centric. I just got calipers from them and the quality control on those was pretty bad. They showed up with new dust boots that were installed incorrectly and a couple of the new seals were even torn. 

 

[xr4tic]

my booster held vacuum until I stepped on the brakes, the idle would raise and the brake pedal was hard to push.

 

I posted a WTB ad on this site, picked up a used one, and it's working great.

[Simeon]

Doesn't Ireland Engineering sell rebuilt ones?

[justadude]

1 hour ago, xr4tic said:

my booster held vacuum until I stepped on the brakes, the idle would raise and the brake pedal was hard to push.

 

I posted a WTB ad on this site, picked up a used one, and it's working great.

Not sure If I mentioned that on my original post but if I do stand on my brakes at idle my idle speed does increase, which would also indicate that the internal control vale is letting atmospheric pressure pass through to the vacuum side. 

 

I talke dot some local BMW mechanics and they told me to was normal to have vacuum boost begin to deplete if you ease on to the brakes for a while before pushing down hard to stop. I am not buying that idea because from my understanding when the brake is in a hold position, say pushed at 1/4 pressure the valve inside should block atmospheric and equalize pressure on both sides of the diaphragm until you push it harder in which case the atmospheric port gets opened again and the vacuum again pull the diaphragm further into the master cylinder. 

 

[justadude]

1 hour ago, Simeon said:

Doesn't Ireland Engineering sell rebuilt ones?

looks like ireland has you send your unit to them and then they send it to somebody else to have rebuilt, a process that could take 3 weeks. I drive my car every day. 

[xr4tic]

IE does sell them, but the rumor is that they are rebuilt by the same place that does the Centric rebuilds.

 

They will sometimes have them in stock, so it's worth checking with them to see.

[justadude]

UDPDATE PROBLEM SOLVED: Logan from Northbay Bavarian took a quick ride in my car and he suspected that the master cylinder had an internal leak on one of the circuits. Strange thing was that symptoms really only became apparent when the vehicle was warmed up like driven for 30 minutes at least when it was cold the brakes felt good. 

 

I took his word and had them order me a OEM master. I am happy to say that the car now stops properly and the brakes have a nice linear, predictable force. In this process I also discovered another small vacuum leak which I fixed so at this point it is really running like a top and stopping great as well.

 

Thats a good feeling.

 

Have a good weekend!