MG Car Repair: Brake Bleeding, caliper piston, rear brakes
QuestionHoward. This is a follow up to a recent question. 1970 MG Midget. Still unable to get hard pedal. Extended the caliper out like you described. Forced the new pad in. Next I put bleeder in rear brake outlet of master cylinder, thus taking the rear brakes out of the equation. Next, pumped the brakes with a clear line from bleeder to master cylinder. Pumped brakes (New speed bleeder in calipers) to get all air out of line. About 200 pumps later, still get tiny bubbles in line. And no pedal. After about 10 pumps the front brakes do lock up. I think I hear air at the master cylinder. By the way, if I put bleeders in both MC outlets I get a hard pedal.
AnswerAndy, air can not enter the system when it is closed except in one place only and that is the master cylinder. But if you got a rock hard pedal with bleeders in both ports of the master cylinder it indicates both section seals are sealing and from then on it is a wheel problem. If you ran a long clear hose from the cylinder bleeder valve up to the reservoir or into a bottle and see air coming out the bleeder then there is that much air still trapped in the lines and caliper cylinders. Keep the cap on the reservoir loose when bleeding and never allow the pedal to come up too fast. Because the spring on the piston can force the piston back in the bore too fast causing a low pressure area in the cylinder that is suppose to draw new fluid down from the reservoir, but the hole is small and the low pressure in the cylinder can draw air past the seal instead. The air vent in the cap is very small and it will try to draw air around the seal first. You can sometimes see this draw when looking at a bubble in your clear line when the pedal is released the bubble may make a slight movement back towards the caliper. keep in mind that fluid will not compress but air will. And the slightest movement of a caliper piston will displace more fluid than a master cylinder piston can displace in one stroke.
Howard
