Brake power booster


Jim Tarantino

New member
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.]
 

Jim Tarantino

New member
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.
 

Capt. Mike

Moderator
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.]
 

Jim Tarantino

New member
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?
 

Capt. Mike

Moderator
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

New member
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.
 

Jim Tarantino

New member
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.
 

Capt. Mike

Moderator
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!
icon_biggrin.gif
 

Jim Tarantino

New member
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.
 

Jim Tarantino

New member
I finally got the chance to get back under the bus yesterday. After some poking around, a re-adjustment of the pushrod, and a check of the brakes at each wheel, I disconnected the vacuum line at the back of the booster again. This time the brakes let go. After plugging up the line at the carb and booster, the brakes function fine. No power, but they stop the bus and return perfectly. I don't know why this didn't work earlier.....So, now is it safe to say that it must be the power booster that is faulty? If so, does anyone rebuild them? It is harder and harder to find one for a 1972. Any suggestions?
 

Capt. Mike

Moderator
VW has never offered any rebuild kits for the boosters. If anybody can, I'd bet it would be White Post. See their topic under Mechanics > USA > Antique restoration . . .
 

Jim Tarantino

New member
Thanks again! btw, I found a vaccum leak and have replaced the intake seals. I'm running a single carb <Holley/Webber 2 bbl.> Now old Carmine runs better than ever. We'll be enjoying rides and camping along the Blue Ridge Parkway this fall. I just need a little more stopping power for those hills.I'll keep you posted.
 

Jim Tarantino

New member
New/used booster still working fine, but after inspecting the last bad booster I noticed there was a lot of corrosion inside under the ruber seals. I was wondering if moisture was entering the booster from somewhere. Since this is the third booster to go bad, I was thinking that maybe moisture is coming in through the vaccum hose. I know the rear hose is from the carb, so no problem there, but the front hose goes through the wheel well and into the body somewhere. I can't tell what exactly that line is connected to. If the body has a leak inside, or collects condensation somehow, maybe it is passed along to the booster and is the cause of the corrosion. Does anyone know the purpose and where this vaccum line is terminated. Again, its the one on the front end of the booster.
 

Capt. Mike

Moderator
Brake fluid is hygroscopic; it absorbs moisture out of the air. This can then accumulate or settle and cause pitting.

Theoretically, the master cylinder & booster are separate but any leaks in the mastercylinder at the piston side can leak back into the booster. Minor amounts will not cause brake failure, but they continued accumulation in an air environment allows that small amount to absorb moisture without limit, but also without any chance of that fluid being moved on or replaced.

The vacuum line should be dedicated and any leaks would cause booster failure, so that is not likely if the brakes work fine.

The mechanical side of the booster -- beyond the vacuum chamber and between the master cylinder, cannot be evacuated of air, but whenever the two are separated, it should be cleaned, dried and a NEW gasket used so there is not continued access to air. If there is a seal surface problem, for example a damaged flange on the master cylinder, that must be corrected to achieve this purpose. Your failures may not be the booster, but caused by master cylinder defects that don't show up in the hydraulic side. It's not unusual to find fluid in the booster during repairs but many don't clean and dry it out, so the remnants continue to attract moisture.
 

Jim Tarantino

New member
Once again, interesting information. Thanks for your help. I guess I'll just keep my fingers crossed that all continues to work well. I did't have any moisture or fluid in the latest replacement booster, and I did use a new gasket, so I should be ok for a long while. Hope to see Ya'all out there bussing this summer.
 

chefboyrdave

New member
Hello,

I am not sure if this is where i am suposed to ask this but I was wondering what thepiece is called that is connected to the hose that goes from carb to brake servo? It is located near the rear left wheel well in my 75 westy. Lookes like some sort of breather. Is it in the manual? I couldn't find the info on it.
Thanks Dave
 

Capt. Mike

Moderator
Transferred to consolidate same topic.

fredo Junior Member posted November 21, 2003 03:27 AM

hello my name in fredo,sorry my poor english, i have been cruising with my 1975, 2.0 liter 2 carb. for over 15 000 miles arond europe. i now have a problem with my power booster, i ve fond that the joint at the very front of the booster is damage. my question is can i fix it some how, i m in no where land to find parts, stuck in the forest with my lap top comp.

is there a danger a run out of brakes. our just push harder on the pedal until i find parts.

tank you very much for your time and support
fredo.
 

Capt. Mike

Moderator
Bentley Section 8-4.5: Repairs are limited to replacement of the filter & damping ring, rubber boot and seal ring.

If you've damaged he seal between the front and the master cylinder alone, you wouldn't have increased brake pressure -- that's just a dirt seal. It appears your damage has also damaged the booster causing loss of vacuum assist, reverting the brakes to the unassisted mechanical mode. They will work, but require an even greater pressure than a non-power assist system.

The loss of assist also means loss of balance (regulator)and reduction of pad to rotor pressure such that regular driving, other than minimum to get to the shop, can be dangerous and should be avoided as much as possible.
 

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