Hi,

Just converted my 65 to power discs. The discs and calipers are single pistons off a 72. Calipers were pretty old, came in a box with the car. Calipers do not appear to leak.

I am using an 85 Vette booster, also of questionable origin. Spent a lot of time and money converting this metric POS to connect to my new 72 Chevelle proportioning valve.

The problem is this: No pressure. I've got an elaborate set of lines that connect the booster to the prop valve. I bled the brakes, but the pedal will not get stiff. Not even spongy. Still goes to the floor with ease. The brakes are barely getting enough pressure to stop the car at idle in drive.

There is a slight leak at the proportioning valve, but it does not seem to be bad enough to rob all my pressure. (does not squirt, just oozes)

Do you think my master cylinder is bad?

If the booster was bad, could I still build pressure by pumping? graemlins/hurray.gif

Any ideas on how to test this without buying a $90 pressure guage?

Also, the vette master did not use a prop valve. In a vette, it was hard lined right to the brakes. Do you think this combo of a vette booster and a 72 prop. valve just does not work?

Jameel,
What master cylinder (MC) are you running? The late model ‘vette MC?

If so you might want to check your brake lines. The MC front reservoir is for the rear brakes and the rear reservoir is for the front brakes. This is opposite of what a stock Chevelle MC would be.

The ‘vette MC you have, as you know by now, uses metric bubble seats as opposed to the SAE flare seats. You could have used a MC from an S-10, which feature SAE flared seats and have the outlets on the same side as a stock MC on a Chevelle.

http://www.chevelles.com/showroom/ProportioningValve.JPG

Make sure you have your combination valve hooked up like above.

Originally posted by jaqazi:


There is a slight leak at the proportioning valve, but it does not seem to be bad enough to rob all my pressure. (does not squirt, just oozes)

Any leaks will allow air to enter the brake circuits, thus you will never be able to build up sufficient pressure. Fix the leak before you do anything else.

Joe

Is this 4-wheel disk? The Vette master will be for 4-wheel disk so if you're only doing fronts you need another master.

For the price, I'd consider relacing the master and the proportioning valve. You can get a rebuild cast master for little and you can get a brand new proportioning valve from GM for about $60.

If the calipers are old and have been sitting around for a while they're probably junk too. They rust in the bore and this will cause the pistons to stick even if they don't leak.

Peter

Jameel,
Check to see if your bleeder valves are pointing up on your calipers. If not you have them on the wrong side of the car and you will have an air pocket in the caliper, which you will not be able to bleed out.

I made this mistake my self. graemlins/clonk.gif I spent a lot of time and money before someone else found the problem. :mad:

Chava

Thanks guys.

I was using a master with plastic booster from an 85 Vette. It has three output lines. Two for the front and one for the back. It was used without a proportioning valve in the Vette.

I had some custom lines made to connect the metric bubble flare fittings to the SAE fittings in the Chevelle prop valve.

I set up the prop valve just as you illustrated.

I took the two front output lines from the master and joined them with a T in order to plumb them to the prop valve. The rear output line went right to the prop valve.

Unfortunately these lines leak on the SAE side.

I tried to save some money by re-using these parts that I got in a trade, but it looks like it is going to end up costing more in the long run. graemlins/clonk.gif

I think the master would have built pressure if I could get the lines tight, but I have become so frustrated with this jury-rigged set up that I just ordered a traditional stock Chevelle booster and master from MP brakes. It was $145 including shipping.

I could have spent another couple of weeks trying to make my own hardlines with conversion fittings, but I came to the conclusion that I was never going to be confident in this set-up.

Hopefully the threads on my prop valve are not too messed up from all the tightening and re-tightening.

Did you bench bleed the master cylinder before you installed it?
I didn't on mine and I got zero pressure too.
Took it back off, bench bled it and way, way better.
Dennis

Originally posted by jaqazi:


I was using a master with plastic booster from an 85 Vette. It has three output lines. Two for the front and one for the back. It was used without a proportioning valve in the Vette.

Hmmm, I've only seen the three output line master cylinders on Fords. Though I don't have much experience with late model 'vettes. And usually the third output brake line is used to provide residual pressure to the rear drum brakes.

Joe

I bled the master on the car. I took the lines and routes them back into the reservior and pumped a few times. Seemed to work.

I actually have the factory repair manual for the 85 Vette. It actually shows two front lines and one rear that route directly from the master to the wheels.