cmdlariat

My master cylinder recently failed on my 1990 Lariat, so I went out and got a new master cylinder. I bench bled the master according to the manufacturer's instructions, took off the old one, cleaned out a ton of crud between it and the brake booster, bolted on the new MC and reconnected the brake lines.

I intended to completely flush the system of all the old brake fluid, which looks like flat coca cola. I had the wife pump up the brakes, hold the pedal down, cracked the passenger rear bleeder and got a very small flow of brake fluid (using a clear 1/4 ID hose) which would quickly stop, then suck air back into the brakes even while she had the pedal down. We repeated this maybe 10 times before I gave up, moved to the drivers rear and had the same experience. It feels like there's a vacuum on the line, so I opened the cap on the MC reservoir, which didn't help.

I don't think there's crud in the lines because I haven't really had any trouble with the brakes before the MC failed. The MC I bought had what I am calling the proportioning valve already installed -- this is the silver octagonal gadget attached to the MC port closest to the firewall. Does this have to be adjusted? Could this thing be restricting flow to the rear brakes? If yes, how do I diagnose and adjust this? Other likely causes?

Thanks for your help!!

coachk64

I know you probably don't want to hear this, but when I had exactly the same problem on my 96 F150 a new master cylinder fixed it.

Sometimes (not often) a new part fails out of the box, especially a remanufactured part. Hopefully it's that simple for you.

Phil63146

You might have a bleeder screw that's full of gunk and not allowing the brake fluid to flow out when bleeding. What I always do is to remove all 4 bleeder screws and use a drill to open the long portion, and to open the side hole of the bleeder where it fits into the caliper or cylinder.

If the brakes are applied I don't know how it could suck air. I highly recommend you inspect the steel brake line running to the rear axle and where they split and run to each wheel cylinder for corrosion.

Or you just might have a bad master cylinder.