Rear brakes are whipping me...
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Joined: Aug 2009
Posts: 333
Rear brakes are whipping me...
This is driving me nuts...
1953 88, original rear, 1965 Delta 88 front, upgraded dual reservoir master cylinder with matching power booster.
Brakes have felt spongey, after replacing nearly everything, so I decided to check it out again. Power braked it, and the rear wheel didn't hold at all.
Jacked up the rear end, put it drive, hit the brakes and watched the rear wheels spin on merrily.
Changed the MC lines to 3/16 (had 5/16 line - don't ask) and bled all four wheels. Fluid at all four wheels, but no pressure at the rear. Pulled the rear drums off - wheel cylinders were not moving at all. Assumed that since I had merely rebuilt them instead of replacing them ($8.99 as opposed to $46.99), perhaps they were now honed out too large and the pistons were getting cocked in the bore; bit the bullet and replaced them. All new parts now everywhere, except for the parking brake spring. Bled the brakes again.
No difference.
Hmm. Pulled the master cylinder off to bench bleed. Noticed that the rear reservoir was not really pushing fluid. Noticed the pushrod had an adjusting nut - lengthened the pushrod. Rear reservoir started pushing fluid. Replaced master cylinder, making sure to adjust pushrod to zero clearance for maximum pedal travel. Hooked bleed lines up, MC looping back to reservoirs, pumped brake pedal, felt much better - lots of fluid travel from rear reservoir now.
Hooked up brake lines, bled brakes - no pressure at rear brakes.
Removed bleed screws one at a time, had someone step on brake pedal and hold it - fluid pours out, does not jet out. Does not dribble, either - there appears to be a fair volume of fluid, just no pressure to speak of.
Only thing I can think of at this point is to take it to a shop and have them pressure bleed the system.
Anyone have any other ideas?
Other details I can think of: I had to redrill the brake pedal mounting lever to ensure a straight-in geometry, so I'm pretty certain the MC plunger is not getting cocked in the bore. When I adjusted the pushrod length, I set it so that when the MC retaining nuts were tightened down, the pushrod was just exerting pressure on the plunger. Should there be any clearance?
I've figured out the whole manual star wheel adjustment routine - this is not an issue. There is simply no movement of the wheel cylinder pistons at all.
During the first go-round with the brakes, I did blow compressed air through the lines leading to the rear - so unless the lines have collapsed, or something, during the intervening two months or so of the car sitting without being driven, the line itself should be clear.
I'm stumped, unless it just really, really wants to be pressure bled.
1953 88, original rear, 1965 Delta 88 front, upgraded dual reservoir master cylinder with matching power booster.
Brakes have felt spongey, after replacing nearly everything, so I decided to check it out again. Power braked it, and the rear wheel didn't hold at all.
Jacked up the rear end, put it drive, hit the brakes and watched the rear wheels spin on merrily.
Changed the MC lines to 3/16 (had 5/16 line - don't ask) and bled all four wheels. Fluid at all four wheels, but no pressure at the rear. Pulled the rear drums off - wheel cylinders were not moving at all. Assumed that since I had merely rebuilt them instead of replacing them ($8.99 as opposed to $46.99), perhaps they were now honed out too large and the pistons were getting cocked in the bore; bit the bullet and replaced them. All new parts now everywhere, except for the parking brake spring. Bled the brakes again.
No difference.
Hmm. Pulled the master cylinder off to bench bleed. Noticed that the rear reservoir was not really pushing fluid. Noticed the pushrod had an adjusting nut - lengthened the pushrod. Rear reservoir started pushing fluid. Replaced master cylinder, making sure to adjust pushrod to zero clearance for maximum pedal travel. Hooked bleed lines up, MC looping back to reservoirs, pumped brake pedal, felt much better - lots of fluid travel from rear reservoir now.
Hooked up brake lines, bled brakes - no pressure at rear brakes.
Removed bleed screws one at a time, had someone step on brake pedal and hold it - fluid pours out, does not jet out. Does not dribble, either - there appears to be a fair volume of fluid, just no pressure to speak of.
Only thing I can think of at this point is to take it to a shop and have them pressure bleed the system.
Anyone have any other ideas?
Other details I can think of: I had to redrill the brake pedal mounting lever to ensure a straight-in geometry, so I'm pretty certain the MC plunger is not getting cocked in the bore. When I adjusted the pushrod length, I set it so that when the MC retaining nuts were tightened down, the pushrod was just exerting pressure on the plunger. Should there be any clearance?
I've figured out the whole manual star wheel adjustment routine - this is not an issue. There is simply no movement of the wheel cylinder pistons at all.
During the first go-round with the brakes, I did blow compressed air through the lines leading to the rear - so unless the lines have collapsed, or something, during the intervening two months or so of the car sitting without being driven, the line itself should be clear.
I'm stumped, unless it just really, really wants to be pressure bled.
Thread Starter
Registered User
Joined: Aug 2009
Posts: 333
No proportioning valve, since it's drums all the way around. Since posting, I've disconnected the rear lines at each point past the master cylinder, starting at the wheel cylinders and working my way forward, and blew compressed air through the lines to make sure there are no obstructions. I noticed that there was very little fluid in the lines until I got to the junction block between the long line going back and the master cylinder. No obstructions in the line anywhere, nice solid air flow, but there sure didn't appear to be enough fluid for the line to be full.
At that point, I had someone hit the brakes for me, and got a nice solid jet of fluid.
I'm going to take it in and have it pressure bled, and see if that doesn't do the trick. It sure seems that the whole problem is just that filling the line by pumping the brakes is far too slow a process. Pressurizing the line should get the line and wheel cylinders full in a hurry.
At that point, I had someone hit the brakes for me, and got a nice solid jet of fluid.
I'm going to take it in and have it pressure bled, and see if that doesn't do the trick. It sure seems that the whole problem is just that filling the line by pumping the brakes is far too slow a process. Pressurizing the line should get the line and wheel cylinders full in a hurry.
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Joined: Mar 2010
Posts: 42
I've always gravity bled empty lines (open bleeders and MC cap, wait till fluid drips out). Then bleed normally. Never been a big fan of pressure bleeders, course I never owned one and have bled brakes hundreds of times without one, so maybe it's just me.
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Posts: 333
Update: Finally got the only local shop in town (family owned for 35 years) to pressure bleed the brakes. His wife reported a stream of "What the hell?" and "That's not right!" noises coming from the shop while he was working on it...
His diagnosis? There's something wrong with the master cylinder, but he's not prepared to say what.
His pressure bleeder hooks into the bleeder screw fittings and forces fluid forward through the lines to the MC. Although he's quite confident there was no more air in the lines, the rear wheel cylinders simply would not receive enough pressure to open the shoes. He was adamant that he could find no leaks anywhere in the lines, that it has to be something in the MC.
I'm pretty certain that the rear brakes were working at one point, but the project has stretched long enough that I can't be 100%. At any rate, I disassembled the MC, looking for anything unusual, and found nothing obvious. I'm waiting for a replacement MC to arrive tomorrow morning, and then we'll see what happens.
I am a bit consoled by the thought that I wasn't just completely incompetent, that my usual stroke of bizarre bad luck was involved. If he had been able to bleed it with no trouble at all, or found something that any idiot should have seen, I'd have been embarrassed...
His diagnosis? There's something wrong with the master cylinder, but he's not prepared to say what.
His pressure bleeder hooks into the bleeder screw fittings and forces fluid forward through the lines to the MC. Although he's quite confident there was no more air in the lines, the rear wheel cylinders simply would not receive enough pressure to open the shoes. He was adamant that he could find no leaks anywhere in the lines, that it has to be something in the MC.
I'm pretty certain that the rear brakes were working at one point, but the project has stretched long enough that I can't be 100%. At any rate, I disassembled the MC, looking for anything unusual, and found nothing obvious. I'm waiting for a replacement MC to arrive tomorrow morning, and then we'll see what happens.
I am a bit consoled by the thought that I wasn't just completely incompetent, that my usual stroke of bizarre bad luck was involved. If he had been able to bleed it with no trouble at all, or found something that any idiot should have seen, I'd have been embarrassed...
Last edited by Erinyes; Jul 9, 2010 at 04:53 PM.
Thread Starter
Registered User
Joined: Aug 2009
Posts: 333
Replaced the master cylinder, and voila, the rear brakes work.
I don't know if I'm paranoid or not, but when I bench bled the MC there definitely was not as great a volume of fluid going to the rear reservoir. After installation and bleeding the wheel cylinders (again!), I finally am getting more normal fluid from the bleeder screws, and the rear brakes actually work.
What I'm curious about is, does a four-wheel drum brake master cylinder have some kind of internal check valve to limit the amount of fluid to the rear as compared to the front? I used a clear hose to bench bleed, and there is definitely fluid movement, but the level of fluid in the front reservoir drops noticeably more (like, three times the volume) than the level in the rear even though the reservoirs are the same size.
What I'm worried about is that the MC was designed for a 1971 GMC and 71 was the first year they had disc brakes. As a transitional year, could they have used the same size reservoir design and tweaked the fluid flow internally? The MC I have on the truck now has had the MC replaced once or twice, and it currently has the standard large front reservoir and small rear reservoir design, so that's no help.
Any thoughts?
I don't know if I'm paranoid or not, but when I bench bled the MC there definitely was not as great a volume of fluid going to the rear reservoir. After installation and bleeding the wheel cylinders (again!), I finally am getting more normal fluid from the bleeder screws, and the rear brakes actually work.
What I'm curious about is, does a four-wheel drum brake master cylinder have some kind of internal check valve to limit the amount of fluid to the rear as compared to the front? I used a clear hose to bench bleed, and there is definitely fluid movement, but the level of fluid in the front reservoir drops noticeably more (like, three times the volume) than the level in the rear even though the reservoirs are the same size.
What I'm worried about is that the MC was designed for a 1971 GMC and 71 was the first year they had disc brakes. As a transitional year, could they have used the same size reservoir design and tweaked the fluid flow internally? The MC I have on the truck now has had the MC replaced once or twice, and it currently has the standard large front reservoir and small rear reservoir design, so that's no help.
Any thoughts?
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