Originally Posted by MY69GOAT

So I've been using the GTO on occasion for little trips here and there for a while now with a slight brake problem. I replaced the master cylinder and left front caliper due to leakage last spring. I bled the entire system for the master cylinder change and ever since then, I've had lousy brake performance. I figured that the previous master cylinder leaked fluid all over the seals for the power boost and ate them giving reduced performance with the new hardware. Not a big deal, just drive slower and give more time to stop. I found out a few weeks ago that the rear brakes are not working at all. Emergency works, just not hydraulics. This is more than likely the reason for decreased performance. Not a big deal either. I figure the proportioning valve is probably messed up or stuck to one side, not allowing pressure to the rears. The big question is, I check the brake fluid (among other things) almost every time I use it and the fluid level is usually fine. By "fine" i mean that there is no noticeable decrease in fluid. Then, every once in a while, the smaller bowl is almost empty. I can't seem to find any fluid leaks. The lines and connections are all dry from the master cylinder to the calipers and rear cylinders (there's no fluid inside the drums either).


The fluid has to be going somewhere. I am not sure about GTO but the old Buick GS I had the brake lines ran through the frame in a certain area, they are bad about rusting in this area and it is very hard to see. If this is not the case the there are a few other things to look at
A. New master Cyl is bad and the fluid is going into the booster. This happens quite often and you will never see it. I would pull it loose and give it a quick look to see if it is leaking.
B. The rear flex line is bad. The line can go bad on the inside and actually act like a flap. If there is air in the line and the flex line is bad it will do all kinds of wierd.
My question is
1) Which bowl on the master cylinder goes to which brake setup? The larger bowl is to the front and the smaller to the rear of the master cylinder. The larger line comes out of the master cylinder between both bowls at the base and the smaller comes out of the protrusion off of the front at the bottom. I believe the smaller bowl goes to the rear brakes and the larger goes to the front calipers but I know that some master cylinders are reversed and they usually have the lines coming out of the master cylinder at the base of the bowl they belong to. I realize this is more of a basic question, but the loss of fluid without a visible leak is making me question my knowledge a little.

You are correct the small is for the rears and large for the fronts.

2) I've never dealt with a stuck proportioning valve before (only heard about it). What's the best way to free it up and get it working properly again?

Do not bother trying to get it to work IF that is the problem however i dought it is. If the valve goes bad it will usuall get stuck and cause the light to come on. The brake light runs off this valve and when they go 9 out of 10 times it will turn the light on. If you determine it to be bad then just replace it, they are not all that expencive. If you do have to replace it go with a adjustable valve. It will cost you a little more money hoverer they are really nice to dial in the brakes.


Also, when I initially bled the system, I got what I believed to be normal flow to the rear brakes. This is why I didn't think about the proportioning valve to start. The pedal has always been stiff. Just as stiff, if not more so, than before the hardware change. I am perplexed as to how at least a little flow would not be enough to make an attempt to stop the rear wheels. I mean, with the rear end in the air, I can free spin the wheels and step on the brakes and they don't even try to slow down. I'm kinda befuddled at the whole situation. Any ideas are welcomed as cruising season is upon us and this new revelation is going to keep me in the garage till I finger it out.