Multiple bad master cylinders - Team Camaro Tech
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post #1 of 6 (permalink) Old Jan 31st, 01, 05:16 PM Thread Starter
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Red face

When I first got my '73 Camaro I did a complete brake job. New pads, shoes, calipers, cylinders, hardware kit, turn drums & rotors, etc. Also, I replaced the master cylinder. Afterwards I thought the pedal felt a little soft, but attributed it to new linings. 20K miles later the front pads wore out. I didn't think I drove it THAT hard, I replaced the pads and checked the back shoes. They looked almost new. So I got the idea the back brakes weren't working as hard as they should be. After running some tests I decided the replacement master cylinder was bad, and put in a NEW one, not rebuilt. Raybestos MC36306. The brakes still felt soft, I ended up replacing the proportioning valve and all hoses. Still running soft, I ran a test, jacked up the back of the car, put it in drive, and held my foot on the brake. This should have been more like a trans stall speed test, but the wheels started turning around 2000RPM. The PARKING brake worked better than the hydraulic. Also, I noticed that bleeding the brakes was difficult. I used Russell speed bleeders, and with these the more I bled the more air would get INTO the lines. It is like, on the return stroke the MC sucks back, and air gets in somewhere. There is no residual pressure valve on these MCs. I replaced the rear cylinders (again) in case it was sucking air past those seals, no improvement. I found that the only way to purge all air from the lines was by a vacuum bleeding. Finally I did this test, I cracked a FRONT bleeder, then step on the pedal, it slowly sinks to the floor. Now a dual master cylinder is supposed to still work in the event of front or rear failure. Reversing the test, cracking a rear valve, the pedal stays firm. I repeated the test with bleeding, then capping just before the proportioning valve, same results. So I decided I had ANOTHER bad MC, and got a replacement. Another Raybestos MC36306. Same problem. Hard to bleed, pedal sinks to floor with front bleeder opened. I don't want to put another MC on my car unless I know it is good. I built a MC bench tester, both the old MCs test bad, I bench bleed them until no air, plug a front or rear port, same results, with front plugged solid, with rear plugged, slowly sinks to end. Yes, I did properly bench bleed the MCs before installing, and yes the brakes are properly adjusted (to be sure I tighten to the point of locking, then pulled the adjuster away and backed off to slight drag). Because there is _some_ pressure the brake warning light does not turn on (unless I crack a front or back bleeder). If you are suspicious of your brakes, because they are soft or backs never wear out, try this. Crack a front cylinder bleeder and see if the pedal stays firm, or bottoms out. Use a piece of tubing, let it run up 6" or so before looping back into the catch bottle, so no chance to suck air back in. If it bottoms out, your MC is bad. So, I wonder, have any of you guys had this problem? And does anyone know where I can buy a quality MC? The local Chevy dealer quotes me about $150. Thanks,

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-ibjoe
now: '73 LT1; '79 Z28
then: '67 coupe; '88 IROC; '95 Z28
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post #2 of 6 (permalink) Old Feb 5th, 01, 05:38 AM
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Location: Ontario, Canada
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Couple of things to check befor you replace yet another MC.
First make sure that the rear shoes are on properly, long shoe to the rear, 2nd adjust the rear shoes, untill you have a slight drag on the wheel as it is rotated, 3rd, make sure the brake cables are not over adjusted, the shoes should sit flush on the top pin there should be no gap,
When bleeding your brakes, jack the *** end up as high as you can get it, try to get the wheels higher than the MC, have some one pump the pedal 4-8 times, while holding downward pressure on the pedal, open the right rear bleeder screw slowly, as the pedal drops, close the bleeder valve, pump and repeat several times, move to the left rear wheel and bleed several times, keep an eye on the fluid level of the MC, check it after two bleeds, leave the *** end high have a beer or two with your helper, and repeat the bleed process,
It sounds like you are having a little hard luck getting all of the air gfrom your brake system, by jacking the *** end high the air in the system will naturaly flow to the rear wheels, thats why you take the beer break,

One other thing that drove us old mechanics nuts, was when they first introduced disc brakes is that compared to drum brakes the pedal always has a softer feel, sitting in shop at a standstill the pedal would always almost drop to the floor, yet I would challenge my customers to try and push the pedal that far when travelling at 20 mph, instant lock up.
I find it hard to believe that you have such bad luck and get that many bad MC's
Good luck
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post #3 of 6 (permalink) Old Feb 5th, 01, 07:01 AM
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I'd be looking for a smashed,kinked,or bent brake line.

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Geezer
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post #4 of 6 (permalink) Old Feb 5th, 01, 08:57 AM
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Maybe try putting some grease around the threads on the bleeder screws to prevent from sucking air in through the threads will help too? Worth a shot anyway.

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Bret Copsey
'68 Camaro base coupe
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post #5 of 6 (permalink) Old Feb 7th, 01, 07:49 AM Thread Starter
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Talking

Problem Solved!
First, thanks to all who replied.
Rob.Canada: Yup, short shoe on front, adjustment was good, brake cable adjustment good. I did the nose dive thing at one point trouble shooting, the front bumper was just touching the ground, while the back was way high. Got the MC level, but that didn't work either. Funny, I always associated disk brakes with a firm pedal, not the other way around. Both my '88 IROC and '95 Z28 had 4wheel disks, and solid pedal.
Geezer: Yup, I did a full visual on all the lines, plus replaced the flex lines w/ stainless steel.
bretcopsey: Yup, I didn't use grease but did use teflon tape.
Anyway, what I did was to buy a genuine Delco MC. $107 from the local store (the dealer wanted $150). I bench bled and tested it first. Installed it and it works fine. On my old MCs, the first one (a rebuilt) fails the bench test, the second one (Raybestos) also fails. With the rear port blocked they go full stroke. The third (another Raybestos) actually tests good. I think I jumped the gun on that one, because the stroke with rear port blocked only is only 3/8" from end on a good MC, I thought it was pegging out when it wasn't.
FYI, here are the stroke measurements on a good MC:
both ports blocked: 1/4"
front port only blocked: 5/8"
rear port only blocked: 1"
no ports blocked (full stroke): 1-3/8"
So you can see if the rear brakes fail the pedal is only a little lower, but if the front brakes fail the pedal is a lot lower. These measurements are right at the MC, they are bigger at the pedal of course, because of the leverage.
Anyway, I am so happy that is finally done! Now on to other things, I can finally put in that rear window defrogger (now that spring is here and I don't need it for a while) and the new plugs and shocks and speakers and all kinda stuff like that..

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-ibjoe
now: '73 LT1; '79 Z28
then: '67 coupe; '88 IROC; '95 Z28
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post #6 of 6 (permalink) Old Feb 7th, 01, 09:58 AM
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sounds like you have a cursed life just like I do.

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