TheSamba.com :: View topic - Explain this to me: sinking brake pedal solved w rear brake adj.

Thu Aug 06, 2020 3:08 pm

My van's (1990 Camper, front discs, rear drums) brake pedal was occasionally sinking after initial resistance. Typically this happened when gently applying the brakes but the pedal would eventually continue to sink. This was most notable during stop and go traffic such as in an interstate backup and I was only slowing the van, not coming to a complete stop. Raising the brake pedal would raise the point of resistance in the pedal stroke and sometimes the pedal would stay there.

No brake fluid had gone missing (the reservoir was still at max and I inspected the lines, rear cylinders, calipers, etc. and didn't see any leaks) so this seemed like a bad master cylinder. After a number of searches here a several comments pointed to rear brakes that needed to be adjusted.

When I removed the drums there was plenty of pad left on the shoes (about 7mm) and all the parts looked like they were assembled correctly. I tightened the parking brake to settle in at 3 - 4 clicks and also did a number of abrupt reverse stops to adjust the rear brakes.

After a 10 mile drive the brake pedal did not sink at all. This included a few mile long downhills gently applying the brakes which seemed to cause the pedal to sink previously.

Is there a chance that the problem is solved and if so, how can rear brakes being out of adjustment cause the pedal to sink? Does the lack of pressure in the rear system cause the proportioning valve to allow the pedal to continue to sink until adequate pressure is reached?

Thanks.

Thu Aug 06, 2020 3:14 pm

Could brake fluid be going into the brake booster?

Thu Aug 06, 2020 3:18 pm

That's a possibility but the reservoir is filled to the max arrow so I don't believe I've lost any fluid. Something to keep watching.

Mon Aug 10, 2020 8:38 pm

Mine does this too even though the "self" adjusters are lubed and free. They were adjusted to rub the drum but I guess they need to be tighter.

Mon Aug 10, 2020 8:54 pm

I had the same symptoms in my rig years ago. I adjusted the rear brakes with the same result as you had- - - that sinking feeling went away.

As for why, I think that with the rear brakes improperly adjusted, there was a sufficient angle in the brake shoe for the self energizing action of the shoe to be over-stated, so to speak, which keeps the pedal high in heavy braking, but can "de-energize" in light braking, which is what can cause the pedal to fall.

There is also the possibility that part of the cylinder of the master cylinder was corroded, causing a leak in part of the stroke, which then causes the soft pedal at that point. Adjusting the rear brakes then keeps the master cylinder piston from reaching that corroded part of the master cylinder, if there is a corroded portion.

Mon Aug 10, 2020 9:07 pm

Howesight wrote:

There is also the possibility that part of the cylinder of the master cylinder was corroded, causing a leak in part of the stroke, which then causes the soft pedal at that point. Adjusting the rear brakes then keeps the master cylinder piston from reaching that corroded part of the master cylinder, if there is a corroded portion.

That's what I'm thinking. The MC piston lip seals are no longer operating in the bad part of the cylinder now after your adjustment.

Mon Aug 10, 2020 9:30 pm

Thanks, all. I've been googling this and have come across a few references to poorly adjusted rear drums being a possible cause of a sinking brake pedal.

Most commonly leaks and bad master cylinders were mentioned but occasionally drum brake adjustment was mentioned, as in 6 out of 7 in this Wagner Brake article:
https://www.wagnerbrake.com/technical/technical-tips/why-are-my-brakes-spongy.html

So far so good, the pedal hasn't sunk since but I'll keep an eye on it.

Tue Aug 11, 2020 4:54 am

i had a vacuum leak that was causing the brakes to not behave. the low pedal issue improved drastically, but the adjusters were not adjusting... they were pretty well worn round so they would not catch all the time. make sure the adjusters have nice edges and spin freely.

Tue Aug 11, 2020 12:56 pm

dabaron wrote: ... but the adjusters were not adjusting... they were pretty well worn round so they would not catch all the time. make sure the adjusters have nice edges and spin freely.

If wheel cylinders don't have internal springs, could that be a factor with OP's issue?

During various types of braking, if shoes are not properly adjusted close enough to drum, would this allow for more movement of shoes at anchor and wheel cylinder which in turn allows shoes to return to a more random position? (maybe this is what, or part of what, Howesight was getting at?)

My worn adjuster wheels likely had about 300,000 miles on them. In hindsight, I'm not 100% certain they were working inconsistently: video I took of the old parts, drum removed, showed the adjuster slipping off teeth. BUT... it then occurred to me that the drum needs to be installed for adjuster to work. In this video test I took, I'm nearly certain new parts are shown. At least it sheds a little light on how the adjuster works.

Since I was rebuilding the rears I replaced adjuster wheel + ends at each wheel.

Neil.

New and old adjuster wheels

Tue Aug 11, 2020 2:12 pm

Vanagon Nut wrote:
New and old adjuster wheels

this clearly shows how the sharp edges get worn over time and what was causing my adjusters to not advance the adjuster bar. one of those "hey i think i figured out why they aren't adjusting" shower moments.

Tue Aug 11, 2020 2:37 pm

Yup. I overlooked those parts over the years, which might be "normal" for most owners. I do wonder if corrosion was a factor in wearing some teeth more than others. Van has lived on West Coast but backing plates did have rust perforation.

All told, I don't think each lever was consistently turning each wheel.

In some ways though, with a failing auto adjust, being "forced" to check and adjust the shoes is not a bad idea. Keeps the drum freed up and an eye on things. e.g. for premature weeping at lesser quality cylinders.

Neil.

Fri Aug 21, 2020 6:18 pm

I'm still having soft pedal with my 1986 Doka. No leaks, rear brakes good, and e-brake good?

Fri Aug 21, 2020 10:35 pm

At some point, I think the only possibility is a bad master cylinder but what do I know, I started this thread. :D

Sun Sep 06, 2020 2:16 pm

My adjuster arm broke off and was grinding inside the drum. Put new shoes, cyclinder on with no changes other than that. Brake pedal was strong until I changed. E-brake was about 20 clicks. AKA a lot.

Did many sudden reverse stops. Now, pedal is super soft for 3-4 pumps then firms up to normal. So I thought I didn’t adjust the rod out far enough. Went back in and spun it out. Same sinking. E brake was 3 clicks and better than ever. After 6 hr drive Driver side drum temp was 120, passenger 340.

Back in and loosening rod. Still soft pedal. Not sure if pads are sitting properly or too high or low or what. Just riding this out till I swap to discs but really am wondering.

Fluid level is perfect. No drips. Filled back up after changing cylinder. Thanks for your thoughts.