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Discussion Starter #1
Converted a 69 to power front Disc brakes, 10 bolt to 12 bolt. All components are new. Bench bleed the MC, bled all 4 wheels. Pedal feels good until the engine is running, pedal goes to the floor. Short rod with a shallow MC, good vac to the booster, lines off the MC, distribution block and pressure valve to the front brakes all leak fluid when opened to check for blockage. I am stumped as is my mechanic. Any ideas what to do next?
 

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What bore MC?

Is the rod in the lower hole on the brake pedal?
 

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What kind of front calipers? Is the bleed valve on top of the caliper? If the bleeder is facing down the calipers won't bleed properly. Sounds like you still have air in the system somewhere.
 

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Discussion Starter #4
Rod is in the lower hole. Calipers are NAPA remans and the bleeder is on top. Now what? Have bled them 3 times and the flow is good with no apparent air.
 

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Do you get a firm pedal if you plug the ports on the master? That would be my first troubleshooting step.

Don
 

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Discussion Starter #7
The pedal is firm without the engine running, however, the rear brakes are not holding but the front are. As soon as I start the engine the pedal goes to the floor when applied.
 

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Your problem is possibly the booster. Either its leaking vacuum, or not enough. The other option could be the rear brakes are not adjusted.
 

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The pedal is firm without the engine running, however, the rear brakes are not holding but the front are. As soon as I start the engine the pedal goes to the floor when applied.
Plug the ports and test. Work your way from there to troubleshoot. Otherwise you are just shotgunning it....

Don
 

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Personally I would bleed the brakes the old fashioned way 1st. Starting at the back brakes. MC still full of fluid after bleeding?
 

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All components are new.
The pedal is firm without the engine running, however, the rear brakes are not holding but the front are. As soon as I start the engine the pedal goes to the floor when applied.
Did you also install all new brake lines (including all of the rubber brake hoses)?
You might want to look at the rubber brake hose that is located just above the rear axle (or any other rubber brake hose that is also routed to the rear brakes) while someone else depresses the brake peddle with the engine running. See if any of the rubber brake lines appears to be ballooning as the brakes are applied.

There could be some kind of clog (or defect) in one of the rubber hoses that is blocking the fluid pressure from reaching the rear brakes. A similar situation happened to a friend of mine. It turned out to be that one of his front caliper brake hoses was clogged and would expand like a balloon when he hit the brakes... causing the brake peddle to go to the floor.
 

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Discussion Starter #12
The rear end, rebuilt came out of my other car, 1500 miles, and worked fine. All hoses and lines are new. This car was a basket case and everything has been replaced except the roof, 1 door and the subframe! I have being doing this for 40 years and have never had issues as I have had with this one. So I must be missing something. I will take all your advise and go at it again. Maybe it just does not want to be restored! I have 2 weeks to deliver it to the client that I am building it for and the Charlotte autofair car show the first of April. Thanks for all your input. . . let you know the outcome. . . . . .
 

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Update. Replaced the MBC, re bled the brakes. . . . no change. All that is left is the booster.
You never answered these questions.

What bore MC?

Is the rod in the lower hole on the brake pedal?
To me it sounds like you have the wrong bore master cylinder. Should be a 1 1/4"
 

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Discussion Starter #15
John, I changed the MBC that is 6 months old from a guy that is building a pro street. I got the whole booster,MBC prop valve and pressure valve from him and am changing out piece by piece. The pedal is in the lower hole. What I have left is the prop valve and booster. I re bled the system a half hour ago and 3 wheels are just spitting and the last one the left front pissed about 4 inches, this is after I installed the working MBC, so after I change the prop valve I will bleed the system again. It can only be those things. Well on a positive note I finished the all new interior and it looks great with no real issues. I like the TMI seat covers and foam but not the door panels, I like the PUI better. Oh well you all have a great evening.
 

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Rear drum brakes?
are they adjusted correctly?
I'd pump the pedal up and hold pressure on the pedal for a minute or two, then look for leaks. Might be a fitting or bleeder sucking air somewhere.
You can do some testing by clamping each brake hose, assuming they are rubber, do one at a time, then trying the brakes while it's on jackstands or lift. I use needle nose vise grips padded with thin aluminum or tape. go easy on the clamping but it's a good way to isolate where the trouble is.
The angled master cyl traps a bubble of air in the MC bore. A good bench bleed should remove it but your symptoms sound exactly like trapped air in the MC bore.
There is a condition where a master cyl is working but when components are replaced and you bleed the system, the master cyl get's stroked farther than it has gone in years. This area of the bore can be corroded and damage the seals on one or both of the MC pressure bores.
 

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Disc/Drum

If so you need a residual valve in the kine to the rear brakes
 

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Your last post said that 3 wheels are just spitting. That is clearly a air problem and not a booster problem. Something us letting air in.
 
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Vega$69;To me it sounds like you have the wrong bore master cylinder. Should be a 1 1/4"[/QUOTE said:
I think the OEM MC piston diameter for power disc brakes was 1.125".
 

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I've been fooling with cars for over 60 years. Never had a problem like this, I am intrigued. Anybody have a fix?
 
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