I am posting here to document the process of my chasing down a terrible feeling brake pedal on my 79 malibu. Car has a 489 bbc/th-400, sn95 8.8 with rear discs. Dual diaphragm 9 inch vacuum booster and larger bore master cyl ( I think from 80's 4 wheel disc fbody, but not sure) Engine makes about 13 inches of Hg at idle. Currently setup with no combo or proportioning valve. 3/16" hardlines from master to tees and stock rubber lines. Front has new cross drilled rotors on stock spindles, and new larger bore gbody metric calipers. Rear has stock mustang rear rotors and calipers.
Bench bled master, 2 person bled, 1 man bled, reverse bled. Times 4. Pushed 2 Gallons of fluid through it. Not getting any air out of it. Tapped all calipers, wiggled all rubber lines. Pedal is very low and won't lock up the brakes. With lines off and ports plugged, pedal is high and hard, with rear rubber line pinched off, no real change in pedal. With front lines pinched off, no real change in pedal. On the original test drive, the front pads hung up and the got warm, the pedal got good and hard during that and would lock them up. I let it cool down and got it home. New aftermarket pads and rotors were overly thick. Pads have now been clearance to not drag. And pedal is still near floor and soft.
Some additional info, a few quick pumps of the pedal will bring it slightly higher. A rev and return of the throttle with pressure on the brake pedal will make the pedal sink (increase of vacuum as rpm drops.) Pressure bleeding fluid from rear bleeders to the master won't move fluid into the reservior, but will bleed out the line cracked at the master, and fluid will bleed from master through rear bleeders. Pumping the brake pedal with engine running, no wheels on, on the hoist, the front calipers stay tight on the pads and don't retract very well i.e. it's hard to turn the rotors. You can push them back with a c-clamp if you open the bleeders but not if you don't.
My current theory, this master has either a built in residual valve or valving that does not allow fluid to return at the needed rate, and does not move the required volume of fluid. Next move, install cast iron 4 wheel disc 79 firebird master with 1.25" bore, bench bleed, reverse bleed the system and test drive. Report back.
If you've read all this and have suggestions, I'm all ears.
Bench bled master, 2 person bled, 1 man bled, reverse bled. Times 4. Pushed 2 Gallons of fluid through it. Not getting any air out of it. Tapped all calipers, wiggled all rubber lines. Pedal is very low and won't lock up the brakes. With lines off and ports plugged, pedal is high and hard, with rear rubber line pinched off, no real change in pedal. With front lines pinched off, no real change in pedal. On the original test drive, the front pads hung up and the got warm, the pedal got good and hard during that and would lock them up. I let it cool down and got it home. New aftermarket pads and rotors were overly thick. Pads have now been clearance to not drag. And pedal is still near floor and soft.
Some additional info, a few quick pumps of the pedal will bring it slightly higher. A rev and return of the throttle with pressure on the brake pedal will make the pedal sink (increase of vacuum as rpm drops.) Pressure bleeding fluid from rear bleeders to the master won't move fluid into the reservior, but will bleed out the line cracked at the master, and fluid will bleed from master through rear bleeders. Pumping the brake pedal with engine running, no wheels on, on the hoist, the front calipers stay tight on the pads and don't retract very well i.e. it's hard to turn the rotors. You can push them back with a c-clamp if you open the bleeders but not if you don't.
My current theory, this master has either a built in residual valve or valving that does not allow fluid to return at the needed rate, and does not move the required volume of fluid. Next move, install cast iron 4 wheel disc 79 firebird master with 1.25" bore, bench bleed, reverse bleed the system and test drive. Report back.
If you've read all this and have suggestions, I'm all ears.
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