The Stovebolt Site Forums The Shop Area Driveline 1st suspect master cylinder?

My '57 2 ton has hydraulic brakes with HydroVac, the booster is frame mounted and divorced from the master cylinder, this is the stock configuration. Currently the brake pedal goes to the floor upon the first push, if I pump it several times it will build pressure and hold a good high and firm pedal. The scenario is the same with the truck running or with no vacuum applied and therefore I do not think the problem is HydroVac related. I adjusted all 4 brakes to obtain a pretty stiff rubbing, no change in the pedal, also the master cylinder was not low on fluid and the hard lines and all rubber were new along with all cylinders rebuilt 8 years ago. I don't want to pull the drums due to the hubs and drums are hard to separate and I would be into wheel bearings, so I want to start with what I think is the master cylinder giving me problems. If the collective minds here think I'm full of bovine manure, then tell me. If you have some suggestions on how to diagnose further, I will try those also. Thank you in advance.

Even though you adjusted your shoes, they need to be tighter. when you pump your brakes multiple times, it’s taking up the slack between the shoe and the drum. If it holds, your master and all lines are good.

I don’t have a big bolt, but I know on my half ton, the shoes drag hard when the’re adjusted correctly. On a big bolt I suspect that drag must be really hard .

I thought the same thing, the funny thing is that one day the brakes were real good and the next the pedal went to the floor. I'll jack it up again and see what I can do for tighter adjustment.

Just tighten them till the wheels are locked, then go check the master cylinder. Let it set a day or so and recheck. I've seen loose seals in master cylinders that let the pedal go to the floor if you step down slow and easy, yet if you stomp on the pedal they hold just fine.

I jacked up the truck today and adjusted the star wheels 'til they were tight, I couldn't spin the tires. The rear brakes have two star wheels and two wheel cylinders, adjusted both. I put the truck back on the ground and moved it back and forth a couple of feet, brakes were almost locked to the point of not able to move it at an idle. No change in the brake pedal, slow easy push of the pedal and it goes to the floor, several fast hard pushes and it builds pressure and stays firm and high. I think my next move is remove the master and rebuild it.

78buckshot - My personal experience with the same circumstances a couple years ago on my 3100 was a failed master cylinder. Under single push steady pressure the cup didn't seal against the cylinder wall, allowing fluid to bypass the piston and cup.

I could rapidly pump it and gain some pressure, but then holding would result in the same pedal drop to the floor. A little different than yours, but thinking it's master cylinder related.

I vote for a master cylinder rebuild or replacement.

+1 on the master cylinder- - - - -the piston seal has probably shrunk enough that it takes a few strokes of the pedal to expand it and actually start moving fluid.
Jerry

Thank you gentlemen for the thoughts based on experience, thank you for sharing.

I removed the master cylinder and found it contaminated. Rebuild kit is ordered, I will flush the entire hydraulic system with fresh fluid.

A little update on the brakes. I cleaned the master cylinder and put a new kit in it. I disconnected the brake lines at each wheel and at the HydroVac, injected alcohol in the main line and chased it with compressed air, injected brake cleaner in the line and chased it with compressed air. I let the truck sit for a couple of days with the lines disconnected to make sure all of the solvent had evaporated. I had the wheel cylinders capped to keep out contaminants. I then put it all back together and did a one man bleed, very solid pedal now with one application, I will do a final bleed when someone other than my wife can help. For now I'm happy with the results. The issue was definitely in the master cylinder.

Great to hear you got it resolve buckshot.

Did you have to hone the master cylinder at all, or just clean it up and install the kit?

When I rebuilt the entire brake system 7 years ago I honed all the cylinders, this time the master did not need a hone job, the brake fluid was cloudy but did not leave any rust or pitting. The very small pin hole in the master had sludge and the check/pressure retention valve has some rust, along with the vent hole in the cap being clogged. Basically just clean-up, new kit, paint. When I removed the hard line from the master to the HydroVac and a small line in the front, they looked like new on the inside. I used copper-nickel when I did the job originally.