Brake Pedal/Master Cylinder goes mushy?

[deronmoped]

I have been trying to figure out what is going on with not having enough "hard" stopping power with my Brakes in my 70 Bus with 73 brakes in it.

What I have right now is the whole shooting match out of a 73 in my 70 Bus. The brake booster and master cylinder are fairly new. The rest of the braking system is in good shape. Fairly new shoes and pads. New spring kits in the rears. New brake hoses. No leaks, recently bled. Rears are adjusted so as to be just on verge of dragging. Drums are original and are within wear specifications.

Driving around the brakes seem to operate normally. It's only when I try to make a hard stop that I seem to run out of travel with the brake pedal and I don't get the performance I should get out of hard stopping. It's feels like, while stopping hard the pedal goes mushy and goes to the floor. With the engine off, the brake pedal is rock hard after a inch of travel.

Could there be something Funky going on with the Master Cylinder? The Booster causing the Master to do something weird? I will disconnect the Booster and see how the pedal feels with a hard stop, see if goes mushy.

Thanks, Deron.

[Bobs67vwagen]

This could be air still trapped in the system, leaky master, leaky booster, or leaky line to booster. I would bleed them again repeatedly to be sure no air is left in the system. I use a mitey-vac one person pistol grip bleeder that works well with no brake pedal pumping and is inexpensive.

[kreemoweet]

With no leaks and no air in the system, pedal going to floor can only mean a failed master cylinder.
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[70bus]

"whole shooting match"

do you have the proportioning valve?
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[timvw7476]

yeah the proportioning valve is tricky. As in with bleeding it prefers to be sitting FLAT on a slab of concrete. If the bus is tilted or up on a jack it is impossible to properly bleed all four corners & chase all the air out.
Can drive you nuts. Flat is where it's at. and gravity bleeding is a great reason to sit & sip something to pass the time......

[lil-jinx]

agreed with gravity bleeding,I put a clear plastic hose on the bleeders ,with a upward tilt ,so they hold any brake fluid comming from the bleeder and it shows bubbles ,when there are no more bubbles ,close the bleeders,the job is done,ensure that the resoirvoir stays full when bleeding.
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[deronmoped]

Yes I have the proportioning valve installed.

What gets me wondering is the pedal is rock hard after a inch of travel with the engine off. This makes me believe the system has no air in it. Once the engine is running the brake feel with the booster working is it runs out of travel (trying to stop hard) and feels soft. I mean I could stand on the pedal with the engine off and it won't move after a inch of initial travel.

[deronmoped]

So in another thread I find out that I probably do not have the correct slave cylinders in the rears, that work correctly without the residual pressure valve. I think I have the 1970 rear slaves without the spring in them. The spring replaces the purpose of the residual pressure valve that was deleted from the brake proportioning valve.

[deronmoped]

One idea I have is, could the bite of the brake shoes and brake pads be compromised? Not broken in correctly, some sort of glaze on them that keeps them from working efficiently. It would be kinda like the feel of brakes that are overheated and even if you stand on them they do not provide adequate stopping power?

[kreemoweet]

[deronmoped]

I took a look at online pictures of the spring used in the rear brake wheel cylinders and it looks pretty wimpy. Doesn't look like it would do much in the way of effect in holding the shoes out closer to the drum after the brake pedal is released.

I did tighten up the adjustment of the rear brake shoes, as tight as you can get them without rubbing too much. Tightening them up more helped a little in the brake feel and stopping power. Still need to do more to get the brakes working correctly.

I'm thinking of adding a residual pressure valve, to see if that helps. If that does not work, I'm going to put new brake drums on the Bus. My drums are in tolerance, but not much wear left in them.

[raygreenwood]

[airschooled]

The star adjustment keeps the shoes close to the drum. The internal spring/RPV keeps the wheel cylinder touching the shoes.

Both affect pedal feel.

Test something for me: after shutting down the engine, press the brake pedal hard over and over. Count how many presses from “problem feel” to stiff. On a properly working power brake setup, you should have a handful of presses before the pedal changes feel and the residual vacuum in the system stops boosting. After that, hold the pedal down hard and start the engine. Describe exactly how it feels over the first few seconds of engine.

Robbie
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[deronmoped]

From what I'm learning, I decided to get new brake drums.

What made me decide to get the new drums was, they were 9.948 on the driver side and pretty much the same on the passenger side. I assume they are over fifty years old and on the verge (.032) of not meeting specifications. That I believe causes the brake shoes to be to far away (not ideal braking performance) from the drum at the wheel cylinder. You can adjust the shoes so at the adjustment end the shoes are in close contact, but that leaves cylinder end needing something to hold them out close to the drum or you end up with too much pedal travel. My problem, too much pedal travel. When I adjusted the shoes to be pretty much rubbing, that helped correct the problem I was having. With no residual pressure valve (VW got rid of) or springs (which probably did nothing) in the wheel cylinders to help hold the shoes out close to the drums, that leads to too much pedal travel. New drums will get the clearance tighter at the wheel cylinders.

I already came across a couple of threads where guys were having similar problems and tried everything and it was finally corrected at the rear end by replacing the drums. I was confused by this, as I always though that the front brakes did 75% of the braking. And that the master cylinder would activate the fronts separate from the rears. But it seems to be turning out that poor performing rear brakes will affect how the fronts work. Well, I'm going to find out when I get my new drums on the rear.

[deronmoped]

As far as the residual pressure valve, 10 PSI to counteract the return springs on the shoes does not seem like enough. And how do they know 10 PSI (what they are selling out there) is going to work on all vehicles. Maybe VW designed the factory RPV to work on a Bus optimally, but then again, why did they delete it.

That and, does the RPV hold pressure when you need it? When you hit the brakes for the first time, did the RPV hold the pressure overnight, over the weekend? Sure on a good newer one it might work correctly. But one that has some years on it, does it bleed off the 10 PSI before the next time you use the brakes?

The springs in the wheel cylinders seemed like a better idea as you don't have to worry about a RPV holding pressure.

Of course these are just fixes to make shoes and drums last longer.

[raygreenwood]