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cluxford 03-10-2017 05:19 PM

More brake woes

C5 Vette disc front end
Original 13" drum rears
7" Dual booster with Corvette Master.

a bit over a week ago I lost all vacuum and got a super hard pedal

The booster has been playing up for a while, works, doesn't work I just never changed it. Now I had to.

Got a new one. Installed it.

Rears lock almost immediately and pedal is so touchy it's crazy.

Appears in the 5 years of daily driving this the booster never really worked. Or not well anyway, as now this thing is so touchy.

Locking the rears ain't fun. So it had no prop valve in it. I installed one over the weekend.

Bled it, drove it, still locking rears and pedal goes literally to floor before it grabs.

the push rod in the booster where it connects to the master is so hard to adjust. If I use the measuring tool, to adjust it, car drives fine for 20 mins then locks the rears and I can't drive it.

So today I adjusted the rear shoes all the way in, so they are so loose that the park brake cannot even be adjusted (runs out of adjustment) for the park brake to hold.

Again cold driving, pedal to the floor then rears lock up.
20 mins into driving, awesome. Solid comfortable just right pedal, not too touchy, and starts to brake with about a 1/2 press on the pedal, front won't lock but they grab before rears and pulls the car up nicely.

This is driving me nuts. I'm at a loss why cold braking has no pedal and locks rears (even with them adjusted so far inwards the park brake doesn't work) but once everything warms up (20 mins of driving) it's perfect.

Anyone ever had this problem.

Only changes, new 7" dual booster and then an adjustable prop valve to rears.

Frenchy666 03-10-2017 06:42 PM

if the prop valve is in backwards could it act like a check valve??
Once the brake fluid is heated and expands it cant return to the master cylinder/reservoir due to the "check" and causes the wheel cylinders to actuate???
or the boosters' air bleed off may be blocked causing air to stay in the chamber??

that is definitely a hard one....

cluxford 03-10-2017 08:19 PM

Prop valve is in right way. And it had same issue before I installed it. I'm already thinking it may be the new booster

chevguy 04-10-2017 06:45 AM


Originally Posted by cluxford (Post 191267)
Prop valve is in right way. And it had same issue before I installed it. I'm already thinking it may be the new booster

Logic says booster since you just changed it, but have no idea how a booster could possibly cause any of the stuff you mention. I would say it’s the master if the proportioning valve is working properly. With the pedal going to the floor are you sure the actuating rod is the right one for the master? If your sure of all this bud I’d be taking a hard look at the master for sure.

cluxford 04-10-2017 07:40 AM

Thanks Ken, yeah I've adjusted the pushrod from booster to MC many many times, both in and out, all the way in and out and many positions in between. No matter what I do the same things occur. After 20 mins of driving, when everything comes up to temp the situation changes. If I set the pushrod for perfect cold braking, once hot it locks the rears. Or I set it so I have no pedal and poor braking when cold, but once hot it's perfect. That's my issue the difference between the pedal and braking capability cold to hot. It makes no sense that it should change under temp
I am thinking it could be the MC has crapped itself in this process. I do have another one handy, from another project (yes it's the same Corvette master). I might throw it on and see how it goes

chevguy 04-10-2017 12:40 PM

Yeah it’s obviously a heating up brake fluid that’s the difference. My money is on your master. I know you know what your doing but make sure the master is disc/drum as well, ie that little restrictor in the port.

cluxford 04-10-2017 02:02 PM

yep it's a disc / drum. What's interesting just drove the car two hours. Brakes perfect the entire time, no difference cold to hot. Stops, actually stop too well. Doesn't lock up rears or in fact fronts, unless I really stamp on it. But get it that way the rear shoes are so loose I have no hand brake. The moment I get the rear shoes where they need to be I'm back to rears locking (note I have no adjustment left on park brake, it's fully adjusted).

Very strange...more investigative work required

350chev 05-10-2017 08:07 AM

Probably completely unrelated to your issue but I had an issue with my rear drum brakes gradually tightening up then locking up during a drive. It turned out to be the flexible brake hose that attaches to the diff - never been replaced and completely deteriorated internally - looked like a completely clogged artery. It would allow fluid to the brakes under braking pressure but behaved like a one way hose, preventing the brake fluid from returning back up the line to the master cylinder. Replaced hoses and problem gone.

Big Vic 09-10-2017 09:23 PM

Have you check your vacuum? Boosters need a minimum of 8 mm of mercury but ideally run at 15mm. If you have low vac could be that when 'cold' you aren't getting any assistance and when 'warm' you are.

cluxford 10-10-2017 03:56 AM

Vic, yep, pulling 18Hg from the get go.

ric28z 13-01-2018 01:58 PM

Might be a bit late but just a thought, early holdens had a part called a reaction disc fitted between the master and booster. basically just a small rubber disc about 50mm dia. These sometimes fell out when mechanics changed master cylinders and the outcome was exactly like your problem.
Don't know if these were ever fitted to other models though

Geoff 4 13-01-2018 08:07 PM

Good point about the reaction disc. Only the Bendix boosters used these, made here under licence by PBR. Holdens, Fords & some Valiants used them. Valiant also used Girling boosters.
The Reaction is was about the size of a 20c piece sits behind the prod that operates the m/c. The prod & seal can be removed to access the disc. As stated, these can fall out. They came in 3 durometer hardnesses to adjust brake sensivity. A great design.

A not so great design is the GM/Delco booster. It uses plastic reaction levers that break, causing on or off braking.

Angry 15-01-2018 12:45 PM

Does M Cylinder have a small valve in the rear brake port ??? caught with one of those...
Id try later slave cylinders in your rear drums closer to the correct size

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