ROUND PROPORTIONING VALVE/FRONT BRAKE BLEEDING 69 Z28

[PACE&Z2869]

Heres an interesting problem i just encountered in the last two days that i thought i would share w/ eveyone in the hopes it would help someone out while bleeding front power brakes.  While bleeding my brakes on my 69 z28 the rear bled out fine, however the front would not bleed.  i continued to get massive air from my bleeders.  I removed master cyl. and bench bled it thinking there was air trapped in master.  There was a small amount but not like what i was getting on the car.  To make a long story short, the problem was the round proportioning/ accumulator valve under the master cyl.  New air was being introduced into the system from the plunger on the backside of this valve.  I hooked this valve up to master cyl. in vice on bench and worked the master cyl. plunger.  Put a drop of brake fluid around plunger on valve and watched it suck in the brake fluid when master cyl. was released.  Air was sucked in every time brakes were let off, yet there was no brake fluid leak at all.  Ordered a rebuild kit from heartbeat city hopefully this will take care of it.  Please respond if any one else has seen this problem.

[SS375HP]

You described in detail the exact situation I encountered while trying to bleed the brakes on a 69SS with power front disks. I too removed the master cylinder and bench bled it. I continued to have a lot of air from the bleeders. Eventually I swapped out the proportioning valve and solved the problem. Let me know how successful your rebuild goes.

[JohnZ]

I haven't had a bleed issue with mine, but I know the valve isn't working; on the first few brake applications when taking the car out, even with light pedal pressure, the car "stands on its nose" instantly, so it's getting front brakes first big-time before the rears come into play. I either need to replace the valve or rebuild it before spring comes, so I'd be interested in how the rebuild kit works out. 

[Farm Boy]

I had the same problem on my 67 manual disc brake car. At first I thought the calipers were sucking in air. I had the calipers sleeved with stainless steel but was still getting air in the front brakes. Rebuilding the master cylinder did not help either. I removed the hold off valve and installed a “temporary” steel line between the master cylinder and the distribution block. The brakes bleed perfect! That was over two years ago and the pedal is still firm. 

If there is a good kit available I would like to rebuild and reinstall my original valve.

[olympic69]

In the Chevy Chassis Service Manual, Sect 5-29 - Brakes -they say the pressure meetering valve must be depressed while bleeding. Could this have anything to do with the air aspiration ( I kinda doubt it).

The rubber in my valve must be worn out, cause it is just about impossible to depress the plunger, and the little dust bootie is worn out.

I look forward to a report on the rebuild- very good news. How much $$$ for the kit?

[GaryL]

Karp's Brake in So. Cal.  (909)985-0800. They charge $65 for a rebuild. I should add they did mine and it works fine.

[olympic69]

...thanks for the Info Ref Karps.

[PACE&Z2869]

UPDATE ON ROUND PROPORTIONING VALVE REBUILD.     I rebuilt round proportioning valve w/ kit purchased from Heartbeat City Shelby township MI. $29.95 + shipping. Brakes bled fine after cleaning and installing this kit. Kit consist of 2 o-rings and a seal. (seal was the culprit) There is another rubber piece inside the valve that wasn't included in kit. (I used the old one). The old seal had a name and P/N on it, and I'll post that later. This seal could possibly be purchased @ local store. 

[olympic69]

Thanks again for the update- good news!!!