Mark71

I pulled my 71 out of the garage the other day and drove it around, probably put about 50 miles on it - longest drive it has had in a while.

I noticed a problem, though. When I press down on the brake pedal, with the engine running and at full operating temp, it comes back up very weakly and very slowly. Sometimes, it doesn't even come up far enough to hit the brake light switch and turn off the lights.

So here are the details:

• Power Disc brakes front, drum rear
• Full overhaul on brakes 3 years ago, rebuilt calipers up front, new lines, new master, new booster, new cylinders rear, new combo valve etc.
• Booster holds vacuum overnight - doesn't bleed down.
• After using up the booster reserve (engine off) the pedal behaves liked I would expect it to (moves only a fraction of an inch and returns pushing the light switch off).
• Brakes work like gangbusters - I can lock 'em up pretty well if I try - and they seem to work evenly...

I've heard that some Chevelles of the same age had a pedal return spring. Do I just need one of those?

Could the booster be bad? It's 3 years old, but has very little actual running time on it...

Could the master cylinder be bad somehow?

Thanks,
Mark

z11375ss

How bizarre. I am experiencing the EXACT same problem. Hunted in the search function to see if there was a post about this kind of thing. I'm taking mine to a qualified mechanic. If you find the problem, report back. I'll do the same. My brakes seem to hang up though sometimes. It's a manual car. So I'll have the clutch in on a slight grade and the car is stuck. If I reach under the pedal and move it up, the brakes release. Thoughts?

Rocketowner

EBay list a brake pedal return spring for '68-72 Cutlass might be wort a try ,but I think I would take a look at the shoes and make sure the return springs there are OK. Larry

joe_padavano

Another possibility is the booster. There is a poppet valve in the center of the diaphragm where the pushrod from the pedal connects. The purpose of this valve is to allow atmospheric pressure into the backside of the diaphragm when the brakes are applied. Once you release the pedal, this valve is supposed to close, allowing engine vacuum to depressurize the backside of the diaphragm and allow the pedal to return. If the poppet valve is sticking or not sealing properly, this won't happen, resulting in poor pedal return.

76olds

It could be a small leak in the Master Cylinder or the booster is failing as Joe mentioned. I believe ther is a return spring under the dash on the manual Transmission cars you can check. Do you hear a whoosh or hint of this sound coming from under the dash ?
If you think it may be a spring return issue, You could lift the rear end up have, someone depressing the brake slowly while you turn the tires.

Eric

Mark71

So something I noticed - it seems to only happen when underhood temperatures are pretty high, like on a longer drive with traffic lights or something. When I first start it up, or drive for short distances, it seems to return just fine. @Joe - could that be temperature related?

@76olds - I have only the typical "woosh" when you jump on the brake pedal. In normal driving, there's no continuous woosh or vac-leak type noise that I can hear.

Oh yeah - I think the return spring is only for manual brakes, at least on Chevelles...
http://www.chevelles.com/forums/40-2...rn-spring.html

EDIT:
I meant to put this link in too. It is the only thing I could find that seemed a little similar:
https://classicoldsmobile.com/forums...s-up-slow.html

z11375ss

I've ordered a new master and will replace mine. Joe posting that convinced me. I'll post back when I get it replaced.

76olds

My vote would have been a small leak in the master cylinder as well. Looking forward to knowing what the fix was.

Eric

z11375ss

I should have it in Wednesday or Thursday. It's not coming until Monday.

Mark71

I picked up a brand new one at the parts store - Raybestos branded. I am going to try to give it a shot this weekend and see if it makes any difference in my case. If it behaves the same way, I am going to assume that I have to replace the booster and MC completely.

I am still wondering if it has something to do with the headers causing higher under-hood temps...

Stay tuned, I'll report back whatever I find.

76olds

[QUOTE=Mark71;936210]

I am still wondering if it has something to do with the headers causing higher under-hood temps...

The under hood temps wouldn't cause any master cylinder/booster issues. Headers heat won't affect any part of the braking system either.
I have headers on my 76 as well.
I think the master cylinder replacement will take car of the low pedal.
The woosh sound that's coming from your booster should be looked at and most likely be replaced.

Eric

Mark71

Hey Eric - I hope you are right on the temperatures. I may have to replace the booster, but they can get very expensive. Unfortunately, rebuild kits for them cost almost as much as brand new ones.

The "woosh" sound, though - it is only when you quickly push hard on the brakes. I think it is just the booster actuating because I hear it on my other (non-classic) vehicles as well. Do you think I need to replace the booster because of it?

Thanks!

76olds

Mark,

Its hard to say without hearing it , how low does the brake pedal go past the gas pedal when you hear the sound?
Where does the pedal return to in relationship to the gas pedal when you take your foot off?
Does the fluid drop in the master over time?

76olds

Mark,
I'm taking off for the weekend now,
I hope Joe P or someone here can help you out a little more.
I have to say even if the booster was in fact done, you wouldn't hear a wooosh sound , and the pedal would be rock hard to push down.
The sound you are hearing in your other vehicle must be something else, heater/a/c door open close, cruise control setting. That is what you would hear in your other car not the booster unless the booster is leaking like the hose/valve is leaking.

The master cylinder is the most likely cause of a low pedal, pumping it back up re-pressurises and allows the pedal to return and the light to go out. Now how much fluid loss is ther ?
That's why I'm leaning more to the master at the moment, the wooosh sound will need to be checked further.
I hope this helps
Eric

z11375ss

Got the Power Booster today. Will replace tomorrow. I will check back in. Wish me luck. This is about an hour process including removing the wiper motor to give me some room.

Mark71

Just fyi, I yanked out my existing master cylinder, drained it and set it aside.

I fired up the engine and just tapped the brake pedal and it returns very slowly without even an MC in place at all. If I shut it down and exhaust the vacuum, it returns very quickly and solidly.

I think I am going to return my parts store MC and buy a new booster and new MC, just because I don't want to mess around with brake parts.

Can anyone tell me what was original to my 71 Cutlass Convertible? There is a single bail, square MC:
https://www.summitracing.com/parts/rsd-dbmc01

And then a dual bail, rounded MC:
https://www.summitracing.com/parts/rsd-dbmc09

Summit lists both as compatible with disc/drum setups and with 70-72 Cutlass models... Thanks!

joe_padavano

Functionally the two designs are equivalent. The rectangular single bale would be optically correct for a 1971.

z11375ss

Well, I started to replace my power booster. I unplugged the vacuum line that runs to it and it made the distinct popping sound of vacuum being released. It holds vacuum tight. The car hasn't been run in at least a week. I'm at a loss to explain my problem now.

joe_padavano

Well, start by going back and reading post #4, then crack open your CSM to chapter 5 and study the diagrams of how the power booster works. All you've done is prove that the diaphragm holds vacuum on the engine side. That is NOT the problem. The problem is that the air valve between the two sides of the diaphragm is sticking and preventing the vacuum from evacuating the firewall side of the diaphragm. THIS is what causes slow pedal return.

Normally there is vacuum on both sides of the diaphragm. A spring in the booster pushes the pedal back. As you press on the pedal, you open a valve that allows air into the firewall side of the diaphragm. With vacuum on the engine side, there is about 14 psi pushing on the diaphragm, which is where the power comes from. Once you release the pedal, this first valve closes and the valve between the two sides of the diaphragm opens, allowing engine vacuum to evacuate the firewall side of the diaphragm. By equalizing pressure, the internal spring can now return the pedal. If the pressures don't equalize, the pedal won't come back.

z11375ss

Thanks for clearing that up for me Joe. I'll have this thing fixed soon.

z11375ss

Ok, now I am just completely confused. I took the booster out of my car and found I had ordered the wrong one to replace it. Rather than have the car down until I could get the new part, I put it back together. I'm under the dash connecting the arm and I pushed on the brake pedal. I thought it felt pretty good. The brake pedal that is. So I get the hood on it and take it for a ride and the pedal is as good as its ever been. I panic stopped the car 3 times and it's just fine. Any comments would be appreciated as to what anyone thinks I did.
As long as I was under there I fixed a clutch pedal rattle that had me a little nuts. Realized after 8 years I just had to bend a piece of metal a little to the left. No more rattle.

z11375ss

While I was out the car completely quit on me. I mean no power to anything. Lights, interior or exterior. No ignition. Nothing. I figured it had to be something I just did so I messed with the underhood lamp wire. That WAS getting power. Nothing else though. I wiggled the wires on the horn relay, looked in the car and the interior light was on. Started right up. Got it home safely. Now I have to clean that terminal and make sure it's ok. Trust me, the damn thing looks new!
Everything on the car looks new. It's the only car I didn't sell out of the collection. Everything else is gone, including the warehouse.

Mark71

Just to follow-up, I ordered a new brake booster from Summit and it got here. Unfortunately, it wouldn't hold a vacuum at all. Summit was cool about it and sent me a new one which I finally got on Friday.

Over the weekend, I installed the new booster and a brand new master cylinder. I bench bled the master and then bled all of the lines (and flushed out all the older fluid).

So far, all my tests indicate that it is working much better now. The pedal returns with some force to the released state. I am not going to call it a success just yet - until I have the time to put some more mileage on it.

I'll report back with any updates. I do really think my old booster was going bad... Thanks for all the help!

joe_padavano

It's possible that you had some debris in the air admittance valve on the booster that was holding it open. When you removed the booster, you likely shook it and the debris got knocked loose. Alternately, the valve may have been sticky and banging the booster around freed it up.