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Discussion Starter #1
I've not had this before.

The past couple of weeks I have had the brakes lock up, not lock up solid, but not release completely.

I seems I can drive for some time fine, brake pedal feels and responds like normal.

Then all at once the brake pedal will feel solid (as in no play at all before brakes engage) and I can tell the brakes are already partially engaged. Just let off the gas and it's clear the brakes are slowing the car pretty harshly without me applying any brake.

I jacked up the car to see what I had at each wheel. With the engine shut off the wheels loosen up quite a bit and spin better. Start the engine and things tighten up again. This tightening up appears to be occurring at all four wheels.

Any more definitive testing I might do?

Does it sound like master cylinder or power booster problem?

What I have for brakes is four wheel power disc conversion. All GM parts: rear Cadillac front Monte Carlo and I believe the power booster and master cylinder are Corvette.

Any advice appreciated.
 

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Agree with above, sounds like the vacuum booster, especially if you say you can crank the engine and the brakes tighten up themselves without you touching the brake pedal.
 

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Discussion Starter #4 (Edited)
Thanks guys, and I get the guess and agreement to the guess. It's my guess as well.

Is there anyone else that might have advice on any additional test I might try?

If there is anyone with brake experience who might have more than a guess?

Anyone know why/how and power booster might act like this. I would be inclined to think if it were worn out/broken (it is about 20 years old) that it would be less effective, not work on it's own out of control.

Memory tells me replacing this power booster is not a terribly difficult job, I installed it about 20 years ago. ;)

My problem might be figuring out what part it is. I don't recall now what year Corvette it was from. I'd imagine though the same power booster was probably used on many vehicles. Probably pretty generic as parts go.

Any chance that based on the PICs below anyone recognizes it's application.

DSC_2206 by Larry Madsen, on Flickr
DSC_2207 by Larry Madsen, on Flickr
 

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Block off the vacuum line, and bleed the air out of the booster by simply removing the one way valve pushed in the booster, and try it when the car is running. They will be stiff, but shouldn't lock up if my theory is correct. I feel the booster is bad like mentioned earlier.
 

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Discussion Starter #7
Thanks for the replies. I had considered pull the vacuum off the booster to insure they don't lock up with no vacuum, have not done it yet. I'll do that.

I assume the "check valve" is the black connector where the vacuum line attaches to the front there on the booster. I can verify that as well.

If anyone has any other input I'll be glad to see it. :)
 

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Thanks for the replies. I had considered pull the vacuum off the booster to insure they don't lock up with no vacuum, have not done it yet. I'll do that.

I assume the "check valve" is the black connector where the vacuum line attaches to the front there on the booster. I can verify that as well.

If anyone has any other input I'll be glad to see it. :)
First make sure your brake pedal is returning. To test when the brakes are sticking lift up the brake pedal with your foot. If you're good there then As mentioned pull the vacuum line to the booster and see if that resolves the sticking.

The check valve keeps vacuum in the booster so the brakes still have some vacuum assist even when the engine is under load and pulling less than needed vacuum for the brakes. Like WOT.

The check valve not functioning correctly will not cause the situation you are experiencing. More likely something in the booster is bad.

If not sticking with the vacuum line removed booster is bad.

Easy to replace. The hardest thing is getting the pin that attached the booter to the brake pedal out. The booster you have looking like a standard 11". single diaphragm
 

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Discussion Starter #9
Thanks John, I appreciate your input.

Yes, the pedal is returning completely. When the brakes lock up the pedal is hard (brakes engages) putting a foot on the pedal has the effect of just pressing harder on the brakes ... no play or slack in the pedal when they are locking up, even though the pedal is all the way returned..

I drove the car to work this morning and drove in such a way as to not need the brakes, just the slightest of touch for a complete stop. The brakes did not seem to lock up at all on my way to work. Driving home I tried the same thing and probably hit the brakes a tiny bit harder a time or two, but still pretty slight. The brakes did lock up on the trip home. My trip to and from work is limited to Flamingo Road between the 95 FWY and Koval (Wynn, about 6 miles) . I stopped to buy gas on the way home and the lock-up eased off a lot when I started back up.

Just got home, I'll change my clothes here and go pull the vacuum and see what I can learn there. ;)
 

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Discussion Starter #10 (Edited)
I pulled the vacuum line off. The check valve is working one direction only, as it should.

I capped the manifold vacuum port and taped over the opening in the booster (to keep crud out).

Drove around the neighborhood a bit. Brakes work, very stiff pedal, but they work. No sticking of the overall system as of now.

I have a couple of days of work on Saturday and Sunday before my days off. I think the car is drivable as is. I'll use it for work as usual for the next two days and see how it acts with the booster disabled.

Meanwhile I'll be looking for the right booster.

Edited to add: This looks pretty close w/master cylinder

 

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I pulled the vacuum line off. The check valve is working one direction only, as it should.

I capped the manifold vacuum port and taped over the opening in the booster (to keep crud out).

Drove around the neighborhood a bit. Brakes work, very stiff pedal, but they work. No sticking of the overall system as of now.

I have a couple of days of work on Saturday and Sunday before my days off. I think the car is drivable as is. I'll use it for work as usual for the next two days and see how it acts with the booster disabled.

Meanwhile I'll be looking for the right booster.

Edited to add: This looks pretty close w/master cylinder

With this just change the booster. Can do easy without having to hassle bleeding the brakes.And you save $60

 

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You can just replace the booster. You don't even need to take the brake lines off. Just ease it out if the way a little. Much easier like John mentioned.
 

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Discussion Starter #13
Yes Keith, got that. I just stumbled onto that combo piece and it looks just like what I have.

My first choice is to replace just the booster. I'll see what I can find locally.

However, my master cylinder is also 20 years old and changing it out and replacing all the fluid might not be a horrible choice either.
 

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Discussion Starter #14
Update and question about specific power boosters at retailers.

I drove the car to and from work today (no vacuum), no issues of any sticking of the brakes ... as I had expected.

I have found that this 11" booster is correct for most Chevy A-body cars. Chevelle, monte Carlo and even some Corvettes applications.
41CMpguC+-L.SL160 by Larry Madsen, on Flickr

The web sites seem to indicate some variation between 4 wheel drum and front disc etc. Does a power booster even know the difference? They appear identical.

Many are re-manufactured and they lose the zinc coating. I'm fine just painting it black.

Input appreciated.
 

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I believe the rod is different.
 

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Discussion Starter #17
Ok folks, I have been thinking this over today.

Some might possibly recall from my prior posts here that my theme on this car has always been to simplify the car. Less things to go wrong or break. I removed the power steering, I eliminated the windshield washer system, dumped the Vintage air A/C and installed the factory heater.

Here's the deal. This four wheel disc power brake upgrade went on the car replacing a four wheel manual drum brake system. Those manual drums were horrible.

As I understand it converting to four wheel disc brakes (even without power) is a big improvement over the manual drum situation. I'd like some input on this understanding, how correct am I on that?

In the name of simplicity I am contemplating the idea of eliminating the power booster completely and going with manual four wheel disc brakes.

If I were to try this, would I need a different master cylinder?

Any and all thoughts on this appreciated.
 

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Yes you would need a different master cylinder. Either a 7/8 or 15/16 bore
 

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Discussion Starter #19
In looking at at master cylinders in the catalogs I have there is no indication of "bore" size.

They do seem to show and application such as front disc/rear drum or four wheel drum. For anything showing four wheel disc I jump to 1982 model year applications.

I also notice a notation of deep hole and shallow hole piston for the rods (which I will need) . However I'm not seeing notation on the master cylinders regarding piston hole depth.

I see rebuild kit showing for 1 1/8" and 1" bores, nothing there on smaller bore sizes (7/8" or 15/16"). I realize I am not looking for a rebuild kit, just noticed the numbers there as reference.

How should I be looking at this while evaluating the offerings?
 

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