SIII master cylender problems

[mbranch365]

I have a 72 SIII that I am attempting to get on the road. The last thing I need to get it driving is working brakes, I have replaced everything including the master cylinder from RN and still can’t get it right. I have bled an entire quart through the system but the peddle still goes to the floor. If I pump one time I get decent peddle but it will very slowly sink to the floor. Has anyone had similar problems? I’m not sure if I have a bad master cylinder or if I just have air trapped in the system.

[artpeck]

I went through a similar exercise. A few thoughts. First try isolating each wheel by clamping the lines. I used a vice grip but I think there is a tool designed to do this. You may get lucky and find out its one wheel. Did you replace the slave cylinders? If so I tracked one of my issues down to a poor quality (cheap) cylinder and ultimately put oem parts on each wheel and have never looked back. Also make sure the shoes are the proper size and snug as you can push the pistons out of the cylinders if there is too much play. Last thing is make sure you follow the proper bleeding procedure working from farthest away to closest.

If you search this forum you will find this is a common issue. My guess is that you have one or more crap cylinders unfortunately.

[NC_Mule]

Getting working brakes (4 cylinders, new rubber lines, MC) was the last thing I needed before my truck was road ready. After multiple bleeding attempts I noticed that when I pressed the brake pedal down then up then down then up I would see tiny little bubbles in my MC reservoir. See if you have anything like that happening. I took the MC back apart and realized that I made an error when I rebuilt it.
pb

[pitchrollyaw]

A friend of mine had very similar symptoms w/ his single circuit 2a. Bled, bled again and replaced his master, same symptoms. The problem turned out to be at the wheels, (I believe) the shoe retract springs were attached to the opposing shoe as opposed to the post on the backplate. Too much tension pulling on the shoes, pushing the pistons too far back in the bores. This description was from a discussion and I have not had to do my brakes yet, but check yours to ensure they are as they should be.

If you can't ever get a good firm pedal it may be a bad m/c out of the box...sadly not unheard of.

[busboy]

Try using a vacuum bleeder but be aware that air can enter the bleed nipple threads. I only crack the bleed nipples and let the vacuum draw the fluid out always closing the nipple before topping up the reservoir. As mentioned start furthest away ending closest to the master cylinder.

[mbranch365]

Thank you all for the advice! I will try again and will take a look at the wheel cylinders and shoes as well.