Brake Booster Check Valve Question

[Hardway]

My '71 240z has a metal check valve with a rubber isolator around it.  It is not plastic or black and grey like the ones being sold now.  I removed it to double check that it was installed correctly and it was, on the booster side of the valve you can blow air through it and on the intake/engine side you cannot.  My question is, how much pressure should it take to open the valve inside? On mine when I blow in to it, it takes some strength to overcome the valve.  When it does I can feel it closing or thumping inside.  It is not smooth but I do not know how it should be.  My car has always stopped fine with plenty of pedal feedback but right now I am working to diagnose why my front brakes are sticking and I am doing a check of all the components.  Any feedback is greatly appreciated regarding the proper action of valve.

[Zed Head]

The FSM says to put about 20 inches on it to test leak-down.  But I can't see how a bad valve would cause sticking.  What kind of sticking?  Side-to-side or stuck on?  People have over-adjusted their master cylinder rods and closed the return hole to the reservoir.  As the fluid heats up  and expands the brakes get applied.

[Hardway]

Both front calipers are sticking.  It has been sitting for 3 days now and last night I took the parking brake off, put it in neutral, and to tried and roll it and it would not budge.  I have run in to this before so I am wanting to do some real trouble shooting before I start throwing parts it.  The check valve may be just fine, I was just curious how it should actually behave.

[Captain Obvious]

I have no direct experience with the brake check valve, but there is a parameter of any check valve called "cracking pressure" and it's the differential pressure required to "crack" the valve open and allow flow. Because of stiction in the sealing surfaces inside, they are never completely smooth and that thump you feel and hear is probably the "crack".

 

As for your calipers sticking... The check valve is a red herring. The brake booster will always have vacuum present in it even when the engine is off. I'm no expert, but I've heard it's a federal safety requirement to allow for continued brake power even in the event of a motor stall. You get like two good pushes before the vacuum in the booster is gone.

 

So I don't know exactly how the check valve should sound, but I can tell you that it has nothing to do with your calipers sticking.

[Hardway]

Thank you for the explanation Captain.  I agree, I don't think the check valve is the cause of my sticking calipers.  The inspection of the check valve is part of my piece by piece examination of the braking system.  Sounds like it is working properly so it will go back on the car.

[Zed Head]

Both front calipers are sticking.  It has been sitting for 3 days now and last night I took the parking brake off, put it in neutral, and to tried and roll it and it would not budge.  I have run in to this before so I am wanting to do some real trouble shooting before I start throwing parts it.  The check valve may be just fine, I was just curious how it should actually behave.

Of course, have to ask, are you sure it's the front pads on the rotors?  I've noticed that my rotors (or discs - why do we call them disc brakes but talk about rotors?) get rusty pretty quickly.  Some surface rust on pads that haven't retracted much could cause sticking, I'd guess.  The piston seals are supposed to pull the pistons back, giving the pads some space, as they return to where they were stretched from.  The elasticity of the seal material is important.  Old or heat-baked seals might lose that quality.  

 

I'm also pretty sure that my rear drums rust up quickly too.  I always get a light grind when I first apply the brakes after the car sits a few days.  Seems to come from the drums though, smooths out right away.

[240260280z]

Another cause that I have read about  is collapsing rubber brake hoses that don't let the pressure bleed off.

 

The other causes (as mentioned above) are:

 

1. corroded pistons that stick

2. corroded disks that stick to the pad

3. warped disks that bind

[Hardway]

The calipers are new and the car drove and stopped fine up until the last half mile of my test drive.  I don't think it is any kind of rust binding them together.  I am going to look at the lines as the behavior of the issue points to them.  I will also be double checking all the caliper hardware to ensure it is greased properly.

[beermanpete]

As a diagnostic procedure; when the brakes are stuck, open the bleeders on each caliper and see if any fluid squirts out. If so, and then the car will roll easily, the pressure is being held in the lines. If not, then the calipers are stuck for some reason.

[Patcon]

What Beerman said ^^^^

[Zed Head]

beerman's test is good for the closed reservoir hole I was talking about in Post #2.  I know you only asked about the check valve so maybe you're on your own problem-solving path.  But you mentioned the brakes being stuck after sitting for 3 days in Post#3, implying that they were frozen after sitting, but now on the last half mile of a test drive.  Implying that they weren't stuck when you started the test drive.

 

beerman's test is for a hydraulic lock in the system.  Rust and pins are mechanical.  I don't see how a collapsed line can hold pressure, unless it was crimped tight, but who knows.

[Hardway]

Good observations Zed Head.  The brakes started locking up on the last half mile of my test drive.  I could tell because it was hard to take off from a stop and the car was being dragged down.  I managed to get in the garage, turned it off, and that has been the last of it.  As I have had time to tinker with it I started with what was easiest to get to.  This weekend I plan to dive further in to it and will start by cracking open the bleeder screws and see if fluid comes out.