'85 Jeep J20: Brake pedal not returning. All tests good, no smoking gun.

The subject is a crusty old Jeep pickup that doesn't get driven often enough for wear to be a real factor.  The system is entirely conventional: front disc and rear drum with factory vacuum power assist, no hydroboost, no trick parts other than braided lines.  All hydraulics were replaced several years ago - not to say that they are all confirmed good, but judging by brake feel and action, they are all doing their job.

The brake pedal is no longer returning to the upright position upon release.  It stays down just far enough to impart a little drag on the brakes and keep the rear lamps lit.

I have power assist, and attempting to wiggle the check valve free of the booster results in a sharp hissing, ergo I believe the vacuum booster holds vacuum and the check valve functions correctly.  If I purge vacuum pumping the pedal with engine off, and then start the engine with pedal depressed, vacuum assist pulls the pedal down farther as vacuum comes in.

When I push on the pedal, I can hear the control valve whooshing as it lets atmospheric pressure in on every stroke.  This is disasppointing, as I was hoping the control valve was stuck and could be freed up with a few shots of throttle body cleaner, but it seems not to be the problem.

This is a dry west coast truck with no rust.  I initially thought the pedal bushing was just sticky, but there is no real change after soaking the pivot with WD40 and working it in.  The factory catalog does not list a pedal return spring.  For now I'm just pulling the pedal up with my toe.  Modern problems, modern solutions, and so on.

It's possible the rear shoe return springs could have broken or gotten soft, or maybe the master is sticky, but the pedal does return and there doesn't seem to be any braking action when the pedal is pulled up.  I'm inclined rebuild the MC in case it's become sticky.  Any other guesses before I just fire the cannon?

Booster spring broken inside?

I've seen them get pretty rusty inside, bad enough that a spring can fail. 

It's easy to take the booster apart and check but it can be a bugger to remove from the vehicle.

In reply to ShawnG :

A broken internal spring is possible, though rust is not typically an issue here. Unfortunately, I was unable to find my borescope to verify through the vacuum fitting.

Disassembling the booster is new territory. Removal should not be terribly difficult on this thing. Not sure where I'd find a spring if it came to that unless we just matched up dimensions at McMaster-Carr. At at that point it might be less headache to simply replace the whole thing.

If starting the engine applies the brakes, you have a bad booster.   That's pretty much your only option there.

Conjecture on if it's a broken spring in the valve or a piece of gunk or crud or other debris is lodging the valve open is kind of academic, as I am not sure you can get replacement bits.

In reply to Pete. (l33t FS) :

My description may not have been clear. Starting the engine does not apply brakes. It just applies vaccum to the booster. I pumped the brakes (engine off) to dump all the vaccum in the booster and kept firm pressure on the pedal while starting; when the engine started, the vaccum boost multiplied my pedal effort and the pedal went down further.

If the pedal is pulled all the way up, it stays up until the next time I push the pedal, engine on or off. It never pulls itself down. 

In reply to DarkMonohue :

You could add a pedal return spring as a bandaid...

In reply to AngryCorvair (Forum Supporter) :

A pedal return spring is probably going to be my semi-temporary workaround. I'm having trouble thinking of anything outside the booster spring that is responsible for pushing the pedal back up.

I added a pedal return spring to my XJ as a temporary measure a while back because the pedal wouldn't consistently return and trip the brake light switch. It turns out temporary can be a long time. Like 8-10 years.