On my '72 Supreme (factory front disc/rear drum), I've recently noticed an intermittent switch between a hard and soft brake pedal. In either situation, the brakes are working fine, and obviously it takes less effort to stop when the pedal is hard. This happens randomly with no rhyme or reason.

I just got back from a drive and checked the temperature of the rear wheels, and they were normal, so I don't think they're sticking, and they don't seem to be dragging.

Any ideas?

 

Sounds like a brake power booster issue. Low vacuum = hard pedal. Dunno exactly what would cause that other than a failing booster or loss of engine vacuum.

 

Vacuum was my first thought too, but why intermittent?

 

Dunno. Can’t troubleshoot something like that without being there in person. Low idle RPM could cause low vacuum, so does the idle drop or stumble when this happens?

 

Not that I've noticed, and like I said it happens sporadically. I can be at a light, take off and get to the next light, and the pedal is hard. Take off again, get to the next light, and it's soft again. Then sometimes it doesn't happen at all. For example, I drove it home from my friend's house last night (a lot of stop lights and traffic) and it didn't happen once.

 

Try lubing the brake rod mechanism/connections

 

Are you talking about down at the brake pedal?

 

Yep. The pedal mechanism. Lube everything that
moves.

 

Ok..and just so I understand, what's the theory behind doing that?

 

I have had two boosters fail on my car in the past 40 years. One would randomly change between a hard pedal and a super soft, way too sensitive pedal and the other just went full hard pedal.

Perhaps verify (or just change) the vacuum check valve to rule that out, and if it still is acting up, I would suspect the booster itself is failing.

 

I was leaning towards the check valve, but even when I have a firm pedal, it still stops fine..no extra effort required on the brakes. Just less pedal travel.

 

Wait, less pedal travel? That's not what you said before:
I thought it was just a difference in pedal effort. The pedal travel should be the same all the time as it's a closed hydraulic system. The only way for the pedal to move further (and therefore push the master cylinder piston further) is for there to be a pressure leak in the hydraulic system.

The difference between soft and hard pedal is a vacuum issue in the booster, or as postulated above, friction (binding) in the pedal linkage.

 

The "theory" is that I have had these "symptoms" happen to me on three occasions in my "extra long" car career. On each occasion, I have solved the problem you described by simply squirting the **** out of the brake pedal mechanism.

 

Master cylinder could be bypassing internally, but not every time you hit the pedal. Seen it a few times in admittedly newer cars but its definitely possible.

 

Sorry for the misunderstanding. Yes, less pedal travel, but no extra effort required to stop. And if there were a leak, then I would see fluid somewhere, correct? And my fluid level would drop in the master cylinder, right?

 

The less pedal travel is the new symptom I'm trying to diagnose. Before, my pedal travel was about 1/2 the distance to the floor. Now (randomly) I just barely have to push on the pedal to stop. Would that be caused what you noted above?

 

Dave - You have an issue with your brake system, in general. You have achieved no resolution pertaining to the incorrect alignment of the differential valve (brake switch) w/in your combination valve (assembly). There is a reason why your differential valve in not aligned & your brake light is constantly illuminated. You need to remove your 50 year old combination valve and replace your 50 year old combination valve with a new combination valve. I am not confident you understand the workings of the combination valve. Simply attempting to get the combination valve differential piston aligned so the brake switch aligns with the center of the differential valve piston is not a fix for a brake system issue. You should review (at a minimum) the many and various components internal to the combination valve. Note in particular - does the brake switch still retain the 50 year old "O-Ring" or has it disintegrated and rust has filled up the chamber including the differential valve piston. Are the springs frozen in place with 50 years of accumulated rust? Is the differential valve piston frozen (e.g. "welded") in place inside the combination valve. Again, it's been stated before - your decision; yet, you have an issue. If you don't want to remove the combination valve because it's too hard to get to......good luck on finding a quick fix.

https://classicoldsmobile.com/forums...9/#post1358684

 

Thanks for the reply Norm. I knew you would chime in eventually 😀 As for your comments, from the onset of my latest issue with my brakes, the directional valve did seem to be the issue, so indulge me in this theory:

Could it be the the valve recently just started to shift ever so slightly enough to where I'm getting equal pressure, and thus, less pedal travel? Then, shift back to off center again, causing the pedal to move more? I should note that if this is the case, the valve isn't shifting enough to turn off the dash light...it remains on even trough the cycle of "more pedal travel" and "less pedal travel."

Also, any recommendations to where I could source a new valve of good quality?

I eagerly await your response.

 

Good morning, Dave. I don't want to get consumed in theory - anything is possible. The fact is you have a misaligned (malfunctioning) differential switch in your combination valve. That issue is not going to resolve until you address the combination valve. Evidently (I would suggest) you don't know if the piston has moved forward or aft inside the valve & you (evidently) don't know what caused the piston to move forward or aft. Again, the fact is it is not centered. The piston is supposed to shift towards the direction where a leak has occurred - this shift then blocks off pressure to the suspecting front or rear brakes. How far it has moved, the direction it has moved, the reason why it has moved & any resolution to realigning the piston is going to require removal of the combination valve.

It is what it is, Dave. You have aftermarket headers installed into a position which makes access to the combination valve difficult. It's going to be laborious - get your head around it. You want it fixed or not? I'd suspect you'd prefer to work on this during the remaining summer months as opposed to the dead of winter - if you're going to perform this yourself. Or, you're going to take it to a reputable automotive service garage. I wish you the best of good fortune - nothing lasts forever. To diagnose this issue you'll need to remove the combination valve - first.

There's a plethora of discussion regarding combination valves on this forum and many other forums. I'd suggest you start by reviewing this thread (below). There are many manufacturers of combination valves for your vehicle. Firstly, remove the combination valve (or have someone do it for you).

https://classicoldsmobile.com/forums...ronado-156137/

 

I need more information. The OP is saying hard vs soft pedal. I understand hard pedal, but is soft like "I have no resistance and am puking brake fluid somewhere" or "I have a functioning vacuum assist now as opposed to earlier." Two different kinds of soft.

 

If the brake light is not on, then the valve is functioning properly. Your issue is either a bad booster, vacuum leak, or heat related by something in the brake system being too close to the exhaust.

 

Ok..maybe I'm not being clear, so I apologize. The pedal is not "firm" or "soft"...I just have less pedal distance (again, randomly).

 

The light is on, and I've narrowed it down to the directional valve being shifted. Plan is to replace the valve.

 

As long as that light is on your brakes will not function correctly if the plunger is shifted.

 

Understood...getting ready to order the valve from Fusick. They also show bracket for the valve...will I need that too?
http://www.fusickautomotiveproducts....?number=PVB712

 

Dave - I don't if the below valve (link) is what you are ordering (or not). If it's the correct one then make certain it comes with the screw-in stop lock brake valve bleeder tool so you can lock down the differential valve piston when breeding the brakes; otherwise, order one elsewhere to ensure you are using it when it comes time to bleed your brakes during the new combination valve installation. Regarding the valve bracket. It would be difficult to determine if you need the bracket without looking at your current combination valve installation. You have headers, right? Will that bracket be required, is there enough room to accommodate the bracket with the headers? Questions we'd be hard-pressed to answer without knowing witnessing your current installation - I think only you can make that call.

GM Proportioning Valve Bleed Tool, AC Delco


Proportioning Valve, 1971-72 Cutlass, 442 with Disc Brakes

 

Eric, you were part & parcel to the previously established condition regarding Dave's combination valve - the reason I posted the previous thread into this thread.

https://classicoldsmobile.com/forums...9/#post1358684

 

That's the one I'm looking at Norm..and I already have the tool. My current valve is mounted on the inner chassis rail behind the driver side tire.. I assume it's the stock location, so does that help with the bracket inquiry?

 

No it would be the exact opposite. Pedal would be normal, but intermittently drop much lower. I misread your post!

 

Here's an image from the 1972 FAM demonstrating the bracket (and, combination valve ASM). Again, I can't answer your question directly since you have headers and I can't see your installation. Did the previous owner need to remove the OEM bracket to accommodate headers? I don't know. "If" it's installed in the original location, I don't see why you couldn't use the same bracket but as said, only you can really make that call. Why not take a better look first to determine if you need to replace the bracket of if the bracket is even installed?