The Stovebolt Site Forums The Shop Area General Truck Talk 41 Master cylinder

Hi,

I have just filled up the brake system with fluid this morning - first time since rebuild - and after bleeding I have fluid coming out of the master cylinder cap when I press the brake pedal.

The fluid goes round the brake system fine and the bleeding went as expected, but as soon as all the bleed screws are bolted up and pressure is applied the fluid comes out of the master cylinder cap.

Is it just a problem with the cap, does it need to be air and fluid tight - or does this sound like another problem, maybe with the cylinder?

Thanks
D

Sounds like you may have just a little too much fluid in the system. Try leaving the level about 1/2" below the cap

That does stop the fluid coming out of the cap, but the pressure applied to the pedal appears to push fluid into the reservoir instead of along the pipes.

I can get fluid out of all the pipes if I loosen the bleed screws one by one but once they are all closed the pedal still goes straight to the floor and the level in the reservoir rises instead of the wheel cylinders expanding.

I may have to remove the master cylinder and put it on the bench to test with fluid and a wheel cylinder Maybe I've made a mistake with the rebuild kit - although I think I followed the manual exactly.

You either still have air in the system or they are not adjusted up right. Those old huck brakes are hard to adjust. When you get them right though the pedal barely goes down. You just apply pressure to it. Make shure there is a small amount of freeplay in the pedal before it starts pushing in on the piston. You may have to adjust the rod a little.

I don't have the drums on yet, I have a friend pushing down the pedal by hand and I'm looking at the wheel cylinders.

I've put a block/stop to the rear brakes and am now just working on the fronts - but just can't seem to get any pressure in the pedal.

I'll try to bleed the fronts on their own now I have them seperated from the rear.

How do you expect to get pressure with no drums installed?

I expect to see the wheel cylinders expand and the brake shoes move when the3 pedal is depressed. When I see that - and I see no leaks from the wheel cylinders - then I can put the drums on and then adjust.

If the wheel cylinders aren't moving at all when the pedal is fully on the floor then fitting the drums isn't going to make any difference that I can think off.

Is the master cylinder rebuilt? If it has been rebuilt correctly then I would look at how it is adjusted. There is a port which in the bore which supplies fluid to the chamber. If the linkage isn't adjusted right, you could be trying to pressurize the system while the port is still exposed. Also you really can't tell much until the drums are on and the shoes adjusted.

Took the master cylinder off the truck and stripped it - again

The Primary piston cup had slipped sideways, probably too dry as it has been sitting for a while after the re-build.

Lots of pressure from the master cylinder now so I'll refit and try again tomorrow.

Thanks for all the help.

I have never heard of trying to bleed brakes without the system being fully assembled. How the heck can you put the system under any pressure when you step on brakes that have nothing to "brake" on? All you are doing is moving fluid.
Joe

He just wanted to see if the breaks were moving, not if they were working!(I think)

You are lucky there was no pressure because you would have blown a wheel cylinder cup plumb out of the hole and have fluid all over your brake shoes. You can't build pressure with no resistance to it.