Master Cylinder not bled

[1968CHEVYC10]

I've been trying to bleed my brakes after replacing all the line, there were a few leaks which slowed things down a bit but I think I've got those fixed. But now I still can't seem to get any good pedal pressure or stopping ability. I was talking to my Grandpa the PO and he said he installed the master cylinder a few years ago but didn't bench bleed it before he put it in. Could this be why I'm having trouble getting pressure? It's a '68 with manual 4 wheel drums. Thanks Yall!!

[71meangreenc10]

Yep....

Got to get the air out of the master cylinder.....


Smitty

[68gmsee]

Yeah, it could be the problem but you could also have defective parts. Check all wheel cylinders for leakage and insure you have good brake shoes. If the rubber hoses have not been replaced, replace them.

Remember that being able to stop the vehicle is the first thing that you need before getting on the road. Master cylinders are pretty cheap at the auto parts and if you suspect it to be defective buy a new one and start over.

[1968CHEVYC10]

would I need to take it off? or can I do it with it still installed? thanks

[1968CHEVYC10]

Originally Posted by 68gmsee

Yeah, it could be the problem but you could also have defective parts. Check all wheel cylinders for leakage and insure you have good brake shoes. If the rubber hoses have not been replaced, replace them.

Remember that being able to stop the vehicle is the first thing that you need before getting on the road. Master cylinders are pretty cheap at the auto parts and if you suspect it to be defective buy a new one and start over.

I have replaced all the wheel cylinders and rubber hoses, did see any leaks on the wheel cylinders, but I'll check again.

[cdowns]

after a few years i doubt that is the issue// it takes me quite a few times of bleeding a system that has new lines to finally get it done // are the brake shoes properly adjusted??

[1968CHEVYC10]

The shoes are adjusted, so I guess I just need to keep at the bleeding? How would I tell if my master cylinder is bad?

[68gmsee]

The way I do it:

I use a section of clear rubber hose that will fit over the brake bleeder valve and place it in a large mason jar or similar container.

Starting with the passenger rear tire, open the bleed valve and have someone step on the brake slowly until it's all the way down and have them hold the pedal while you retighten the bleed valve.

Do this several times as needed making sure the master cylinder stays full of fluid. If it runs out you may have to start over.

After all of the air is completely out go to rear driver side and repeat process.
Next go to passenger front and then driver side front.

Sometimes I go through almost a large can of brake fluid.

If after you insure that there is absolutely no air in the system, depress the brake and keep pressure on it. If it holds, you're okay. If it sinks down slowly, your master cylinder may be leaking internally.

[Shyguy]

The master cylinders that have gone bad on me were pretty obvious, the peddle would just go to the floor with little if any resistance.

Does the peddle feel spongy, usually means there is still air in the system?

A lot of guys try a gravity bleed, seems to work for them. I usually take the manual out and go by what it says to do, since my memory is not as good as it use to be.

I have bleed master cylinders on vehicles before, but it is kind of messy, a lot easier to do it in a vice. I use to have some old lines that had bends on them so I could bleed the master cylinder by pumping the fluid back in the reservior.

[rusty baker]

All the new master cylinders come with hoses and instructions for bench bleeding.

[Shyguy]

Originally Posted by rusty baker

All the new master cylinders come with hoses and instructions for bench bleeding.

Good info, shows you how long it has been since I replaced a brake master cylinder,lol.

[cdowns]

you should start your bleeding from the rear drivers side brake as that is the one furthest away from the master cylinder

[mbgmike]

Just remove the line and install plugs to test the cylinder. if the pedal drops its bad. if it holds firm its good. You don't absolutely have to bench bleed it but its the recommended way. some shops will bleed them on the vehicle, been done for years

[1968CHEVYC10]

I went ahead and took it off, didn't really know what I was looking for at the time so I'm not sure if it had been bled or not, but it's bled now and I'm about to go put it back, I'll let yall know how it goes.