Brake power booster

[Jim Tarantino]

I have a 1972 Westy and I've replaced my brakes power booster three times. twice with used parts, and once with a new Brazillian booster. In every case the booster has siezed up after a short while. The others became completely frozen, the current one sticks a lot, but is still working as long as you pull the brake pedal up with your foot. I also replaced the master cylinder this last time, as well as the brake fluid. Does anyone have an idea as to why this keeps happening, and is there an easy fix to my current sticky power booster problem?

[This message was edited by Capt. Mike on May 31, 2002 at 01:25 AM.]

[wildbill]

have you checked the vacuum to the booster?

[Jim Tarantino]

W O W ! Thanks for that detailed reply. I'm sure you don't get paid enough for all of that! I've got some work ahead of me, but since leaving the bus parked for a couple of days, I'm rejuvinated. Although this next camping trip may be cancelled for now, I have resolved to get this matter repaired no matter what. I'll read your comments again, pull out the manual, and get psyched to battle the brake beast. I thank you very much for all of your help. It may be a while, but I'll be back with an update.

[Capt. Mike]

When the obvious -- in this case the booster itself -- doesn't appear to be the cause, it's time to go back to some basics.

Hydraulic brakes work on the simple principle of pushing a fluid, which can then be done with great mechanical advantage (pedal length vs. cylinder throw) into a receiving cylinder that reverses it to something mechanical. Power brakes simply use an assist to further increase the mechanical advantage of the pedal.

It appears your are getting plenty of 'out' pressure but it is either not releasing or the mechanical side does not generate the spring return pressure to return the hydraulic fluid back to the master cylinder/reservoir side. The 'return' pressure does not have that huge mechanical advantage the other way. It's source is the brake return springs and relies on there not being any hydraulic resistance since the released pedal now has the compensating port in the master cylinder in a free return stage. Therefore that master cylinder, even a new, could be defective -- perhaps a piece of debris from manufacture or rebuild blocking the compensating port.

If you've now eliminated the master cylinder, pushrod adjustment and the booster as sources, follow the flow downstream.

From the master cylinder, the fluid SHOULD be going direct to all metal lines. Are any of these crimped or possibly corroded? Damage that gives them a tiny out portal that works under that great mechanical advantage may be so small as to make the return so slow that the brakes appear locked (but might eventually release with time). Within the metal lines will be "T's" to split the lines to L & R, one front and one rear. Like any junction, this becomes a possible source of debris interupting return flow.

Then, from '71 on, it passes through a rear brake pressure regulator. This is designed to keep 'out' pressure low enough so as to not lock up the rear brakes in a panic stop. It proportions the front to rear since the fronts do most of the stopping. Without it, the tendancy of the rears to lock up and the van to swap ends gets scary! It can be tested via the Bentley 8-5.1, but is not repairable.

From there it passes, still through all metal lines, to the wheel suspension mounting area where it connects to a short, flexible line. The lines allow the suspension to work. This can be a weak link as they are more susceptible to damage and old age. Older hoses have been known to get soft, deteriorate from the inside, or have been damaged by contamination. The inside can get spongy, swollen and even contaminate the fluid through flakes of hose or gummy residue. Picture a garden hose that gets that characteristic swell or bubble; that can occur both ways and is more common internally on a brake hose since their design is for protection from external damage and 'outward' pressure. Some recommend these hoses be replaced on a regular basis, say every 5 or 10 years. I've never had a hose failure, but I attribute much of that to changing the fluid every 2 years with premium DoT-4 so water and contaminates don't get the chance to attack the hose lining. Typically, these hoses don't fail together, so I ask are you sure that the lock up is both wheels equally & together?

Next in line is the rear brake cylinders. They too work on the principle of tremendous force outward, and the return spring force working against no back-pressure. Also not likely to fail together.

Finally, you have the mechanical return side that has two components. The return spring is meant to pull the shoes away from the drum and push the fluid back through the brake cylinder to the system. Shoes are designed with what used to be called self-energizing, which means the rotational force of the wheel actually helps hold the shoe firm and flush with the drum. Within the brake assembly. There are several areas where a the shoes slide over the backing plate and also must slide smoothly in the pistons slots of the cylinders and the star-wheel adjuster. A grooved drum can itself become a source of return resistance. Combined with self-energizing, the shoe may be locking out and not able to return due to mechanical problems. I recommend every shoe replacement include a full overhaul or replacement of the star-wheel adjuster and NEW springs. VW sells hardware kits for some models.

This is also an area where aftermarket shoe sets may not exactly match OE. I've even seen the shoe metal body itself not match OE as aftermarket vendors attempt to use shoe bodies that fit several applications or cheap copies.

The 2nd part of that mechanical equation is the parking brake. The parking brake contains pins & arms that will move with the foot brake action on the shoe, but should do so freely and not hinder return.

Some of that 'return flow' concept also applies to the booster. Is the vacuum assist stopped with pedal pressure removed? i.e. Nothing to act as a defective check valve that allows the booster to add boost but not release it? I mention that because your one attempt at cutting of the boost supply appears to have worked.

All these things ought to keep you out of trouble for a while!

[Jim Tarantino]

Thanks again! I'm better off than I thought. Mostly because this website is awesome, and your help has been that too! Thank you again, I'll let you know how I make out. I have a state inspection coming up, so it will have to be right.

[Capt. Mike]

I would investigate the pushrod adjustment (Bentley 8-4.1). This is critical to the valve function. The compensating port clearance function is described there; you may not be allowing the brake fluid free passage and thus 'blowing' or locking up the booster with excessive pressure. This manual override may just be forcing fluid around the master cylinder seals.

[This message was edited by Capt. Mike on June 13, 2002 at 03:44 AM.]

[Capt. Mike]

The "Bentley" is the official VW Factory Shop manual. Called that because they are the publisher for VW. See photo top left of every page in the site bookstore banner. That's the one that's pretty much a requirement for responses on the site -- see Guideline #2.

[Jim Tarantino]

Yes, I have replaced the vacuum lines because the old ones were cracked.That brings up something very important that I forgot to mention before. When the second booster failed I was on a steep hill with my foot hard on the brakes. When I attempted to start off the brakes were completely jammed. You couldn't pull up on the pedal with both hands. I removed the vacuum line to the carb, plugged it on the carb side, and the brakes worked well enough to get me home. There was no power assist, but there wasn't any sticking either.

[Jim Tarantino]

I have made the adjustment on the pushrod and unfortunately my brakes are still jammed. I read the chapter you suggested but I don't know if there is something else I need to do. If I did block the compensating ports with too much pressure, do I now need to bleed the brakes or do any thing else? In response to a previous post, I have replaced all the vaccum hoses and clamps, so we know they are OK. The master cylinder was replaced, and as I mentioned this will be the third power booster that I need to replace. Does anyone rebuild them? It was a sorry day yesterday, I really thought about selling the old guy. I am at the end of my rope with this problem.

[Jim Tarantino]

Thanks Capt. Mike. I'll try that adjustment, however I don't have the Bentley book....But I do have the VW shop manual. I'll check there. Now, just to be sure I know, you don't think the master cylinder seals are damaged, do you?