Cutlass Touchy Brakes

[Magolds84]

Hello all,

I have an 84 Cutlass Supreme 2 door. Ever since before I bought the car, the brakes are extremely touchy. It's at the point of when you're coasting at a slow speed and apply the brakes, the front end dips down and it'll lock up the tires some. That's with minimal pressure applied. I feel it's more with the front brakes that at the rear. It's also definitely worse when you first start driving. As everything warms up, it calms down some, but still super touchy. It looks as if the proportioning valve has been replaced not too long before I bought the car. Rear shoes are newer. Front brakes and rotors are worn down but not to the point of needing changed yet. Any similar issues from anyone? Any advice on where to start to correct this is greatly appreciated.
Thanks!
Mike

 

[KGB]

Gonna chime in here because I'm dealing with a similar issue. I've replaced all the brake parts on the front wheels. Rotors, pads, calipers, and the rubber lines at the wheels. Still the brakes would lock up. Drove into a shop about 4 months ago and when I hit the brakes, they locked up. Got out to see the skid marks at the front wheels, and they were at the back instead. So I bought all new rear brake stuff. Drums, shoes, hardware, self-adjusters, link bar, wheel cylinders. If it came off, it was replaced with new parts. After everything was back together, adjusted and bled. If I stood on the brakes, the wheel would spin as someone tried to tighten the lugnuts. Up until now, that had fixed the brakes grabbing and locking up. Yesterday my brakes have gone back to consistent pressure for the first 10% of pedal travel, then at 11% my brakes feel like they're at 89%, and sending me through the windshield. I'm pricing out a new master cylinder and brake booster, as well as a proportioning valve as I type this. Those are about the only parts left on my system I haven't changed or repaired, aside from the hard brake lines themselves. Not sure what help that gives you, but I feel your frustration. I've been chasing this now for almost 6 years. I've taken it to a shop a few times, and every time it decides it's not going to lock up for them.

 

[malibudave]

From the factory, the 1982-1988 G-bodies came with LOW drag calipers and a step bore master cylinder. LOW drag caliper retract the piston into the bore a little further than a normal caliper. This is done with a beveled piston seal.

Rebuilt calipers, from what I can tell, are rebuilt to NON low drag or normal specs as the part numbers for these calipers are all the same from 1978-2004. A NON low drag, or normal calipers, uses a square faced piston seal that does not retract the piston into the bore as far as a LOW drag caliper.

A rebuilt, NON low drag, or normal caliper will work with any master cylinder. A LOW drag caliper can only work with a step bore master cylinder. If you put a LOW drag caliper on a car with strait bore master cylinder, you car will have a long, low pedal stroke and it may run out of pedal stroke before the car can stop.

A step bore master cylinder has two bores for the front brakes only. It has a larger bore initially that provides the volume needed to get the front LOW drag caliper's piston and pads back up against rotors. This bore then transitions down to a smaller bore to provide the line pressure and clamping forces on pads and rotors to stop the car.

Maybe, if you use NON low drag calipers on a 1982-1988 G-body with a step bore master cylinder, you might run into the situation you just described. The step bore master cylinder is supplying more volume than the NON low drag caliper needs initially, so when the step bore master cylinder's large bore transitions down to the smaller bore, it applies a lot of pressure suddenly giving you the touchy brakes you describe.

I would suggest a strait bore master cylinder to remedy the issue. A newer master cylinder option is a RIGHT HAND drive, 2000 S-10. Part number is RAYBESTOS MC390572. This master cylinder has a 1.0" bore and the correct outlets sizes to work with your brake line fittings.

NOTE: A normal, LEFT hand drive S-10, has the 1.0" bore, but the outlets are on the passenger side of the master cylinder and the rear brake port is different than what came stock on the 1978-1988 G-Bodies. DO NOT USE THIS MASTER CYLINDER unless you want to replumb your lines.

You can also go with a 1978-1981 24mm bore master cylinder, but these are usually cast iron and unreliable.

All the years below used the same calipers and all the part numbers are the same for these years for these calipers.

1978-1981 "G-Bodies" came from the factory with NON low drag, or normal calipers and a 24mm strait bore master cylinder.
1982-1988 G-Bodies came from the factory with LOW drag calipers and a 24mm/36mm step bore master cylinder.
1982-1992 F-Bodies came from the factory with LOW drag calipers and a 24mm/36mm step bore master cylinder.
1982-1997 2WD S-10 Trucks and Blazers came from the factory with LOW drag calipers and a 24mm/36mm step bore master cylinder.
1998-2004 S-10 Trucks and Blazers came from the factory with NON low drag, or normal calipers and a 1.0" strait bore master cylinder.

1978-1981 - NON low drag, normal calipers and strait bore master cylinder.
1982-1997 - LOW drag calipers and step bore master cylinders
1998-2004 - NON low drag , normal calipers and strait bore master cylinder.

 

[Magolds84]

Thanks for the info guys! The more I think about it when I drive it, the brakes do not apply as soon as i touch the pedal, but about an 1/8th to a quarter of the way down, then it's like all the pressure is applied then. It surely feel like the front to me, especially how the front dips down the way it does, but obviously could be something with the rear as well. So, I do wonder if it has something to do with the master cylinder. I guess I'll start with the front brakes and go from there, since they are getting close to being shot.

 

[KGB]

Yes, thanks for the info Dave. I've got an email out to the manufacturer of my calipers, to see if they are low drag or not. Then I'll go from there. Magolds84, my regal felt the same way, and sorry for kinda hijacking your thread. With the front dipping down when the issue happens. It started out small, feeling like the brakes were sticking when going from a stop. Then it got to the point that every time I stopped the car, it felt like the front passenger wheel was locking up. Turned out to be the rear passenger wheel instead. Doing the rear brakes, stopped them from physically locking up. But the pedal inconsistency, and super touchy-ness is still there. So in my case, the only things left to replace are the master cylinder, booster, and proportioning valve. If your brakes do start to lock up Magolds, find a big parking lot and get someone to stand outside the car and tell you which ones are ACTUALLY locking up. Instead of just being an idiot like me and throwing everything and the kitchen sink at it.

 

[malibudave]

Yes, thanks for the info Dave. I've got an email out to the manufacturer of my calipers, to see if they are low drag or not. Then I'll go from there. Magolds84, my regal felt the same way, and sorry for kinda hijacking your thread. With the front dipping down when the issue happens. It started out small, feeling like the brakes were sticking when going from a stop. Then it got to the point that every time I stopped the car, it felt like the front passenger wheel was locking up. Turned out to be the rear passenger wheel instead. Doing the rear brakes, stopped them from physically locking up. But the pedal inconsistency, and super touchy-ness is still there. So in my case, the only things left to replace are the master cylinder, booster, and proportioning valve. If your brakes do start to lock up Magolds, find a big parking lot and get someone to stand outside the car and tell you which ones are ACTUALLY locking up. Instead of just being an idiot like me and throwing everything and the kitchen sink at it.

The manufacturer will most likely say they are low drag, but they are not. Low drag is thrown around as a marketing term now.

If you have a touchy pedal, usually it's the master cylinder bore size. Usually its the bore size being too small, but in your case its both the bore size and, I suspect, the step bore design.

Your booster seems to working well, I would not change it unless it goes bad. A bad booster gives you a hard pedal and no stopping power.

You will need to increase the bore size of the master cylinder. I would go with a 1.0" bore to start with from the 2000 Right Hand Drive S-10 and see how your brakes react. If you are still getting premature rear lockup, it would then change out my prop valve to a disc/disc one.

 

[Magolds84]

All good KGB. That's going to be my next step. If I end up getting the car out this weekend, I'm going to verify front or rear for sure first and go from there.

 

[Texas82GP]

I'll defer to malibudave as his knowledge in these areas is obviously beyond question.

Still, I'll throw this out there. Some of these cars came with cast iron proportioning (combination) valves that suffered rust issues. Many were replaced under warranty with brass valves. I'm wondering if this could be the problem for either of these gents?

 

[Magolds84]

I'll defer to malibudave as his knowledge in these areas is obviously beyond question.

Still, I'll throw this out there. Some of these cars came with cast iron proportioning (combination) valves that suffered rust issues. Many were replaced under warranty with brass valves. I'm wondering if this could be the problem for either of these gents?

I know for sure that the valve on my car is brass. I didn't replace it, but I believe the owner directly before me did. Definitely looks on the newer side.

 

[KGB]

Still, I'll throw this out there. Some of these cars came with cast iron proportioning (combination) valves that suffered rust issues. Many were replaced under warranty with brass valves. I'm wondering if this could be the problem for either of these gents?

I know for sure that the valve on my car is brass. I didn't replace it, but I believe the owner directly before me did. Definitely looks on the newer side.

And I couldn't even tell you what mine is. I think I glanced at it once when I was doing the engine swap, and never looked at it again. I'd planned on taking a look at it last weekend while I was doing a new rad and trans. cooler. But things went a little sideways and that didn't happen. I'll take a look at it this weekend, and if it looks obviously pooched, I'll replace that first. But I'm thinking I'll start with that right-hand S-10 master that MalibuDave talks about, and a new prop. valve once I get paid.