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Old 03-10-2017, 05:19 PM
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More brake woes

Buick

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.
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  #2  
Old 03-10-2017, 06:42 PM
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Frenchy666 Frenchy666 is offline
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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....
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Old 03-10-2017, 08:19 PM
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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
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Old 04-10-2017, 06:45 AM
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Quote:
Originally Posted by cluxford View Post
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.
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Last edited by chevguy; 04-10-2017 at 06:48 AM..
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Old 04-10-2017, 07:40 AM
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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
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Old 04-10-2017, 12:40 PM
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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.
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Old 04-10-2017, 02:02 PM
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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 ......to 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
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Old 05-10-2017, 08:07 AM
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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.
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Old 09-10-2017, 09:23 PM
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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.
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Old 10-10-2017, 03:56 AM
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Vic, yep, pulling 18Hg from the get go.
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