Intermittant front Disc Brake Dragging

[GusC2it]

I had to fix a V Dub that way too. I should have remembered that. I wonder if old or cheap fluid could exacerbate the problem?
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[kuleinc]

I may be buying and towing home a 71 Westy with this same problem. If the rod adjustment doesn't fix the front brake binding I will try a booster shim... Will report back prolly later tonight...
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http://www.thesamba.com/vw/forum/viewtopic.php?t=496946 1987 VW Vanagon Westfalia with 1.8T

[shiningstar76]

I've been battling the same problem for months. Was going to try new calipers.
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[kuleinc]

It seems, after my bus sits for some time, and you initially push the brakes they push like normal, and feel normal, but if you hold the pedal down, it slowly gets closer to the floor, and then when close to the floor stops. If you lift your foot and reapply the brakes, it now goes down a normal amount and stops and holds and feels great, until you try to move the bus. It takes just about every amount of power the engine can muster to move the bus, with your foot off the brake pedal... wierd.
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http://www.thesamba.com/vw/forum/viewtopic.php?t=496946 1987 VW Vanagon Westfalia with 1.8T

[SGKent]

[kuleinc]

I'm going to buy a master, and some new hoses and fluids. I took the reservoir out behind the seat since it was empty and had gunk in it and will clean it out and then let it dry for several days before I put it back in... The P.O. has a clampy set of pliers on the front right brake hose for some reason!?
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http://www.thesamba.com/vw/forum/viewtopic.php?t=496946 1987 VW Vanagon Westfalia with 1.8T

[Synapse]

Sorry to bring up an old post but......

Would you guys who have used a shim between the master and brake booster care to share what you used, how you measured how much shim you needed and where you got it from? I believe this is where my issue lies as well.

I'm having the issue of all 4 brakes sticking after a few miles. I have a 78. I installed a new MC, brake booster, all new hard and rubber lines, rebuilt calipers, new rear slave cylinders, all new pads/shoes/rotors/drums, new vacuum hose from booster to engine, and new rubber sleeves for the resiviors. The brake linkage is adjusted per Bentley.

Thanks

[busdaddy]

Hmmm...... anythings possible, so you're sure there's some freeplay between the pushrod and pedal?

Remove the master and the O ring and carefully hold the master back in place feeling for when or if it touches the booster rod, the gap will likely be how much shim you need. Whatever you use for a shim it'll have to seal against vacuum, paper gasket material parhaps?
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[Synapse]

Thanks for the reply Busdaddy. To answer your question: I did indeed make sure that there was some freeplay. Everything was followed exactly as outlined in Bently.

Anyway, I believe I have localized the problem. This afternoon I unbolted the master cylinder from the booster. As I did so I heard and felt the brakes release. So I took the master cylinder and brake booster completely out. What I found was that when the master cylinder is seated to the booster, the push rod is already engaged.

Here is a picture with both the MS and booster out of the car, unbolted, and with the booster piston fully seated:

As you can see, with the booster piston resting against the seat of the master cylinder there is 1 to 2mm of distance between the housing of the booster and the master cylinder. It is now obvious to me why the brakes were always tight as the booster has always been partially depressing them even at rest.

The question is did I get a poorly rebuilt booster or a poorly machined master cylinder.

Here is a picture of the booster showing that the piston sits about 2mm proud of the booster. Does anyone know if this is normal?

I dont have a good picture of the master cylinder but it has 47.68mm of travel from the edge of the guide to the bottom of the seat inside (sorry don't know the technical term). This places it squarely flush with the edge where it would mate with the booster.

I've decided I'm not going to mess with shims and instead am going to replace the offending part. I'd rather not replace both just to hedge which one it is and would rather only replace the wrong one. Does anyone know if the piston on the brake booster should be proud of the housing? How about the depth of the seat of the master cylinder piston?

Thanks again.

[busdaddy]

Got the old master still? How does it fit?
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[Synapse]

Unfortunately I pitched it. That's a good thought though. I think from now on I'll hold onto old parts until I confirm the new parts fit.

[kreemoweet]

@ synapse: I take it you are measuring the depth of the hole in the front piston of the M/C. I get just about the same 47.7 mm
as you did, on a spare Varga (brazilian) M/C I have on hand. I don't have the tools to get an extremely accurate measurement
there, because the hole bottom is rounded. I think the most natural and useful reference point to take measurements from is
the booster/master cyl. flange joint. With respect to that, the pushrod seat in the M/C is about .016" inside the booster, so there
would indeed be a problem with any booster whos pushrod isn't a tad shy of emerging from the booster.

It would be interesting and helpful to get some measurements from folks who have boosters and M/C's lying around. Apparently
this is getting to be a fairly common problem with bus brakes.
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[Synapse]

Thanks Kreemoweet. That gives me another data point to compare mine to. I did a google search and looked through the classifieds here. Of the boosters I can discern, it looks like the piston is not supposed to be exterior to the rim of the housing. I'm going to start by getting a new booster to compare. I'll report back my results when I have some.

[Tcash]

Hi, make sure the pushrod in the booster is not adjustable. If it is not you need to shim to achieve the correct booster pushrod clearance. I have not seen the shims in a long time. But a good parts house should be able to source them. Here is a pdf. Good Luck
http://www.autoshop101.com/forms/brake05.pdf

[Synapse]

OK, I have some results to report as promised. I was pretty fed up with the booster and the remanufacturer so I decided to just order a new booster from those folks across the pond that sell new ones. While I was awaiting it to arrive from its sailing voyage I decided to dissect the old one.

I was pretty paranoid after reading about the giant spring in the booster so I took some precautions. First, I built a jig to hold the thing securely in my 20 ton shop press. Then, I snugged it down with the press and pried the two pieces apart still snug in the press. After that I built a completely sealed enclosure surrounding it out of 3/4" plywood with 2" woodscrews and reinforced with 2X4's that would withstand any unexpected occurrences while slowly letting off of the pressure of the press. As I released the press I had a good laugh at myself. Even though the booster was under some pressure, I could have easily done it without the enclosure or the press. In fact, I can compress the spring and whole shebang with my hands. If I ever decide to do that again, I'll probably just sit on the booster, pry it apart, and then stand up. With that said, I would never use a booster I rebuilt and don't recommend anyone else do that either.

Anyway, here are some pictures of the cadaver for future reference if you ever need:

[Synapse]

So I waited a couple of weeks and my new booster arrived in the mail. To my shock I had the exact problem as before. I almost fell over. And then I had a closer look..........

It turns out that the tip of the piston of both my new booster and the rebuilt one are removable. Not only that but there was a spacer placed in between the tip and the piston.

Note: some have a threaded adjuster on the end of the pushrod. That can be adjusted with a wrench.

Here is a picture of the piston. On the end you can see the spacer and tip:

Here is another view:

Here is what the piston looks like when you remove the tip and the spacer:

Here is a picture of the tip and spacer removed. They just pull out with some pliers:

It turns out that by removing that spacer, I had a perfect 1mm of play between the end of the piston and the cup of the master cylinder. Problem solved. All I needed was a little circumcision. I've been living in braking bliss ever since.

I hope this helps someone in the future. Hopefully, you can learn from my initial lack of observation. I suspect the engineers put this tip on to be replaceable in case of wear and the spacer to accommodate different master cylinder depths. Anyway, good luck.

Edited
Tcash

[busdaddy]

Thanks for posting that, not only the dissection but the spacer deal, now that you mention it I've encountered a few that needed the master shimmed away from the booster but now I'll look for the washer first
Any chance of some close ups of the sealing surfaces the 2 valve discs sit on for future reference?

BTW better safe than sorry regarding the precautions.
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[Jeff Geisen]

If the master is shimmed away from the servo I would suspect the system would lose vacuum, no?
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[busdaddy]

[storm]

I too had the same problem. Front brake locked up after pushing brake petal. Release petal and still can’t move front wheel. So I loosen the two nuts on the MC, heard a sound and the front wheels moved. So I got two washers measured .010 insert them between MC and servo. Now every thing works. With engine running and push on brake petal and released. Note everything was replace except hard lines. All new. ‘71 bus. Can anything leaked pass the MC and servo? Air or brake fluid? I know to removed the pushrod spacer would be better, but I can’t move MC back enough.
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