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Discussion Starter · #1 · (Edited)
Trying to finish the rear end swap in my 55. Put in a 99 GMC Jimmy 4x4 with disc brakes. Front brakes are 12" disc/calipers from CPP, with a 1 1/4" mc bore, 8" booster. Have bleed the brakes several times, several ways (gravity, pump the pedal method and with a hand held vacuum. I can get a good brake pedal with the engine off. As soon as the engine is cranked, the pedal will go to the floor...Any suggestions as to what can cause this? :1zhelp: Thanks, Bryant :shakehands:
Another question: I noticed in all ads for Master Cyl/Prop valves for disc/disc, that the FRONT line from the master cylinder goes to the FRONT port in the prop valve and the REAR to the REAR. On my old master cylinder/prop valve for disc/drum, the FRONT line from the mc went to the REAR port of the prop valve. The question is, do I run the lines front to front or back the way it was (front to back)?
 

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Almost every GM plumbing setup I've ever seen is front/front, rear/rear.

Sometimes on a disc/drum setup it's obvious, as there's a big reservoir and a small one. The big one is for the discs, no matter whether it's in front or back. With disc/disc or drum/drum though, they are the same size.

No one ever convinced me there is anything in the average master cylinder that's different front and rear. That's all done in the combination valve.

Back to your air in the lines. Did you bench bleed the m/c? Is there any preload on the master cylinder from the booster pushrod? Is there any preload on the booster from the pedal linkage? Preload will keep your master cylinder from getting a full charge of new fluid when you release the pedal, and that will keep you from fully bleeding the brakes.

Are your bleeder ports at the top of the calipers? Can you get good flow from each bleeder when you bleed?
 

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Discussion Starter · #3 ·
Almost every GM plumbing setup I've ever seen is front/front, rear/rear.

Sometimes on a disc/drum setup it's obvious, as there's a big reservoir and a small one. The big one is for the discs, no matter whether it's in front or back. With disc/disc or drum/drum though, they are the same size.

No one ever convinced me there is anything in the average master cylinder that's different front and rear. That's all done in the combination valve.

Back to your air in the lines. Did you bench bleed the m/c? Is there any preload on the master cylinder from the booster pushrod? Is there any preload on the booster from the pedal linkage? Preload will keep your master cylinder from getting a full charge of new fluid when you release the pedal, and that will keep you from fully bleeding the brakes.

Are your bleeder ports at the top of the calipers? Can you get good flow from each bleeder when you bleed?
At first, did not bleed the mc because it never ran dry of fluid. Do not feel that there was any preload in either case. It worked very well as a disc/drum. All bleeder ports are in the "up" position and the flow is "ok" on the left rear and very good on the right rear and both fronts. Going to make me a bleeder tool like the one David has built so I can bleed the system again. This time with another master cylinder. Thanks for your reply.
 

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Discussion Starter · #5 ·
Still not on the road.....

After solving several small leaks at the new prop valve, I have no leaks in the brake system. The pedal is high and firm with the engine off or the booster vac line disconnected. Soon as car is cranked, pedal goes down close to floor. Read where a disc brake system needs a minimum of 16 inches of vacuum to work properly. I ran a vacuum gauge to the port on the intake just behind the carb to see what vacuum I was getting.......Only 12 inches at idle. It goes up to about 20 inches as the rpm's go up. Is this the reason my pedal is low or do I have a bad booster? After the car has run for a few minutes, I pull the vacuum line loose at the booster and it makes the noise as air is coming out like it is suppose to do. I have not driven the car as yet, not sure if it stops or not. Trying to figure out what my problem is. Anyone have any suggestions?
I'm open to all ideas........Thanks, Bryant :shakehands:
 

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Bryant, the 12-20" vacuum is not uncommon and is sufficient unless you have a very small booster (7" or maybe a single diaphragm 8").

Check out your booster per Pops' link. Also look at the booster troubleshooting procedure in the tech section at mpbrakes.com. Similar to the CPP writeup, but you never know when there's a tidbit, clarification, or added info that helps clear things up.

From what you wrote I think you have a booster problem but don't take my word for it, go through the troubleshooting procedure.

If the booster checks out, you may want to go to a dual diaphragm 8". Consider that for your replacement if yours is a dud, especially if don't or can't get warranty relief.
 

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Thanks Guys for the advice and info. I'll let you know how it turns out....Bryant
:anim_25: Bryant....Be interested in knowing what you find.
 

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Discussion Starter · #12 ·
Tested booster

I checked/tested the booster this afternoon as per the CPP and the MP charts. The booster passed the test or did in my opinion. I called CPP tech support back and Van recommended that I "block" off the rear brakes and see what happens. I did and no change. He suggested that I "block" off the front brakes and see what happens. This I can't do because I have braided brake hoses on the front. So I guess its back to bleeding the system. My experience with bad boosters is that the pedal becomes very "hard'" when it goes bad. The pedal seems a little "mushy" and goes down but not all the way, to the floor. Can turn car off and pedal comes right back up with just a light pump or two. This has just about got me and I''m about to run out of ideas. If bleeding again does not do the trick, I may go with the 8" dual diaphragm unit as suggested. Then the question arises, should I go with a "complete" system of Booster/MC/Prop Valve or just a Booster? The complete system would be a match set as apposed to a mixed set. What do you guys think? Thanks again, Bryant :confused0024:
 

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Discussion Starter · #13 ·
Order new booster......

Even though the booster checked ok, I'm still going to replace it. It suppose to be here Saturday. Got the old one off and ready for the new one. Sure hope this IS, WAS, the problem,
 

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Have you checked your brake pedal linkage and your booster to master cylinder pushrod for preload? If you have preload, you can't bleed the brakes properly, as the master cylinder piston doesn't retract far enough to get a fresh charge of fluid.

I suppose too that you could have a leaky master cylinder.

You can block your front brakes by disconnecting the lines and plugging the ports with plugs you can buy at the auto supply. You'll have to bleed the front brakes at the plugs after installing them. This would tell you if you had a leak at the front piston of the master cylinder. You've already checked the back, kind of. I assume you pinched the hose with vise grips or a C clamp to do that.

But, the initial symptoms you posted still somewhat point to the booster.
 

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Discussion Starter · #15 ·
What to do now....?

After replacing the master cylinder with a new 1 1/8" bore for disc/disc, a new 8" dual diaphragm booster, a new disc/disc prop valve, new rear calipers, rotors, pads, hoses, I now have a good firm pedal that only drops very little when the engine is started. I can feel "two" different "contacts" when depressing the brake pedal. at the top very close together. The brakes do not grab or hold the wheels from turning after applying the brakes. However, when coming to a stop, the car just does not seem to want to stop as good as I expected it to. You are not "pulled" toward the dash when doing a hard stop. I'm puzzled and would like some of your ideas as to what to do next. Thanks, Bryant :shakehands:
 

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Bryant,

I guess the old saying "misery loves company" is true in this case as I'm going through the same thing... I don't know whether to maybe pipe in on your thread or start another! I, too, found that pinching off the rear brake line the pedal was about the same and I went up to the local auto parts store to inquire about a different master cylinder and they said they couldn't even guess at what to use for the conversion. What was the application for the 1 1/8" bore master cylinder you now have? I am really leaning toward the MC as I had originally put on a front disc conversion and kept the rear drums...

Justin B.
 

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Discussion Starter · #17 ·
Same brake issue

Bryant,

I guess the old saying "misery loves company" is true in this case as I'm going through the same thing... I don't know whether to maybe pipe in on your thread or start another! I, too, found that pinching off the rear brake line the pedal was about the same and I went up to the local auto parts store to inquire about a different master cylinder and they said they couldn't even guess at what to use for the conversion. What was the application for the 1 1/8" bore master cylinder you now have? I am really leaning toward the MC as I had originally put on a front disc conversion and kept the rear drums...

Justin B.
Justin, sorry to hear you have a similar problem. The master cylinder came from Jeg's and is a GM style with the 1 1/8" bore. I have read since the installation, to use a 1" bore with 4 wheel disc. Not sure if this will solve my problem or not. If I had one, I would try it to see. Hopefully some one will come up with a solution to this situation. I know I can't be the only one to have or had this problem. Thanks for your input and hope you can solve your situation very soon. Let me know what you do to solve it. By the way, have you replaced anything as of yet? Bryant
 

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All I have replaced during the troubleshooting has been the rear "intermediate" flex hose which was probably almost 60 years old! I have, up to this point, just installed the new rear disc kit and bled the hell out of it.

The tech at Right Stuff Detailing listened to what I had tried and tested and the issue does not seem to be in the rear so he suggested to put plugs in the MC outputs and if the pedal was high and hard the issue was in the calipers or air in the front. I replaced the front calipers (late 80s full size Chevy) less than a year ago so I'm doubting they are the issue...
 

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All I have replaced during the troubleshooting has been the rear "intermediate" flex hose which was probably almost 60 years old! I have, up to this point, just installed the new rear disc kit and bled the hell out of it.

The tech at Right Stuff Detailing listened to what I had tried and tested and the issue does not seem to be in the rear so he suggested to put plugs in the MC outputs and if the pedal was high and hard the issue was in the calipers or air in the front. I replaced the front calipers (late 80s full size Chevy) less than a year ago so I'm doubting they are the issue...
How do you like the Right Stuff brake kit. My local classic chevy dealer recommends their kits.
 

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I think your next best bet is to build or buy a power bleeder. I got to the same place you're at with my brakes and the power bleeder is what made the difference between "OK" brakes and very good brakes.

One quick question though. Where do you have your brake clevis rod mounted on your brake pedal? A lot of places recommend a hole 1" below the stock mounting point with power boosters (this will reduce your leverage). If you have moved yours down, you might try putting it back in the stock location to see what happens. Of course, if you have one of the brackets that angles the booster up for correct alignment with the lower mounting point, you'll need to change the bracket in the long term, but you should be able to at least test it on the stock mounting point temporarily.
 
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