On my 1968 roadster, the front calipers were leaking, I did a rebuild on them. The front fluid level was low but not empty, I filled before touching the brake pedal. The rear part of the system was not touched.
The pedal went all the way to the floor with no resistance. My first question is what happened to the safety factor of the dual system?

Under the master cylinder is a "pressure differential valve, brake warning switch.

Is it possible that this is stuck, causing a pressure drop to rear ?

I bleed the front brakes, and got pedal, and also bleed rear brakes, made sure the rear pistons were not stuck, and verified they were moving against the rotors.

After driving the car , it stops, but doesn't seem like I'm getting full action ( I doubt it would slide on pavement) In gravels, I did a slide, and all 4 wheels did slide.

Has anyone seen this before ?

Thanks !

 

The valve thing that you are talking about can be rested by hard (heavy ) breaking. When I redid my brakes on my 79 the peddel never felt right after I bleed the system, so when I took the car down the road I slammed on the brakes and the peddle got frim.
From what I understand the valve is design that when you loss your breaks it cut off the brake fluid from going to the rear so that you always have front breaks.
I hope this helps and that you get your brake problem fixed.

 

This has been covered over and over on this forum. There is NO hydraulic connection between the front and rear systems. When front or rear fails with a leak, the rear or front will still stop the car, albeit not as well. The switch's, usually erroneously called the proportioning valve, only function is to alert the driver that there is a problem with the brakes when there is a pressure leak.
If, for instance, a hose going to a front caliper is severed there will be NO front braking, the rears will do all the stopping, the BRAKE light on the tach will illuminate, alerting you to DO SOMETHING to the brake system.
NOTE: The heavy duty brake system, identified by having two pad retaining pins on the front calipers, does have a real proportioning valve that limits fluid pressure to the rear brakes.

 

I believe the larger section of the master cylinder is usually the reserve for the front brakes, i could be wrong though.

 

Pete, No matter how many times you repeat this, you'll still be wrong. No, it isn't a proportioning valve, but its function is more than just turning on a light. It also serves to block off the low pressure (leaking) side of the brake system. If you don't believe me, I suggest you open a rear bleeder, jam on the brakes until the light comes on, and then see how much flow you can get from that bleeder. It will be a trickle if anything. I've done this.

Now of course this begs the question, why do you need such a device in a braking system where the front and rear circuits are isolated? I can only imagine that the GM engineers were thinking of the MC piston seals going bad and leaking from one half to the other. In that situation, you no longer have 2 separate hydraulic circuits, and a leak at any corner could potentially drain the entire system if not for that shutoff valve.

@ the OP, your shutoff valve likely has blocked off the front circuit because of the pressure drop the leak caused. Is your brake light on? To reset it, push the pedal down as hard as you can with an idling engine. Hold it down until you feel a thump in the pedal. That's the piston sliding back to center. If that doesn't work then you might need to pull it off and recenter it manually or replace it. (Or do what a few us have now and just plumb it out)

 

Pump up the brake pedal and hold the pressure for 30 seconds, if the pedal goes down than repair the M/C. If the pedal stays hard try bleeding again. PG.

 

 

You can do that from a caliper bleed screw as well.

 

Good info on the valve, guess I missed the previous discussions. Thanks for reposting the resetting method.

Bill

 

That's a good find

Makes sense too but I've never seen the switch returned to center this way.

 

Well I guess if I'm wrong, this is wrong too

Para. 6-1-2.

Anyway the point I was trying to make was that this statement made by J.T. Vette was wrong.
Here is a photo of the valve disassembled:


Cheers,
Pete

 

Could be your both correct, on my 74 the "Dogbone" (the part in the middle that trips the light switch) has a spring on each end. Once a front caliper leaked, and when I steped on the brake the light would go on and when I released the brake pedal the switch would go off. Each time I pressed on the brake more fluid would leak out of the caliper. A year later a rear caliper leaked and the same thing happened, press the brake pedal and the light came on, release the brake and the light went off, and the more I used the brake the more the fluid would leak out. Maybe some switches will stop the fluid flow but mine will not. PG.

 

Try reading it. It supports what I'm telling you. It is not just a light switch.

Actually no, he is correct. It will shut off the flow to the low pressure side.

Thanks, I know what it looks like. I have mine out. I've verified what I'm telling you both in and out of the car. Look carefully at the diagram you posted. What do you think is going to happen if that piston slides all the way left or right?

 

Here's what mine looks like.
http://s243.photobucket.com/albums/f...rakeswitch.jpg

 

I'm pretty sure mine wasn't spring loaded, but I'll go check. I wonder if a frustrated owner put those in after the fact? Either way with that piston out of center (and the light on), you're getting now flow or reduced flow to the lower pressure circuit. You could prove this out via a bleed screw.

 

Now that is interesting ... yours specifically says that the light will only come on with the pedal depressed. Yet there have been quite a few people like myself and the OP whose light will stay on and we also get the leaky circuit blocked off. Additionally there's a documented GM procedure for recentering the switch. Maybe you're onto something. 2 versions of the switch? A self-recentering one and a pain in the azz one?

 

Wow, a lot ofopinions here, they all soundrihtonewayor another. I will describe why i started to look on the forum, you will see why..... Ok, when i press my brake real hard, the brake ligh comes on.... Do Some chkg and the rear resevoir is low, i fill it aand in a short time, the cycle continues. All of the calipers look dry, the engine lines etc. The rear cross member that the cross brake line is on is wet. Found it! What pete described is what happens, press extra hard i am being notified by a light to look. Needs fluid. On a side note, i called andrew at muskegon brake to get some part info, he described the proportioning valve as pete described it..... No matter, i like the fact that it will isolate a problem. Ok, here is the question, in the process of removing the rear cross line, one fitting after another rounded, so i started to just cut them off. I loosened most everything so i can see what the re-assembly needs to look like. I will replace from the proportioning block rearward, what is the bes way to fish the long line thru the frame rail, and would you guys mess with the rear calipers or just put the lines on. I am just not a brake guy, but i am a quick read! Thx much

 

Just wanted to say that I,m old in years but new as owner of a 1974 corvette. And very new to this forum . Started with the brakes because that's where I,m at. Got some good help already and plan on seeing me a lot because I have just got started on this life time dream restoring an old vett,

 

Welcome blue money pit..... You will get a lot of help on the forum! Btw, to get the new brake line over the cross member, i gut it and put a union in the line...... Way way easier. Looks fine, works fine.