Tewski

I replaced the master cylinder, hard lines, flex lines, and all calipers. I benchtop bled the master cylinder and all was fine. I installed the master cylinder and attached the hard lines. The fronts are fine, but I cannot get the rear master cylinder reservoir to remain primed. It won't pump any fluid when the line is attached. I removed the line and plug the MC with my finger and had the wife pump the pedal, after a couple of pumps, its pumps fine. I remove my finger, attach the line and again it won't pump. Loosen the fitting, and its pumping air. I've tried this multiple times but can't figure out where to go from here. Any thoughts?

Thanks

Peterbuilt

When you have the rear line disconnected and press on the brake pedal the fluid comes out but when you let the brake pedal return you are sucking air into the M/C.

Rebleed the M/C.

With the front line connected and the rear port plugged the pedal should be rock hard.

If not then the rear seal in the M/C could be bad.

MelWff

have you tried gravity bleeding in the rear, all 4 bleeders, no pumping

DUB

I would try this...because if your rear lines and calipers have a lot of air in it. Using the 'pump' method will take a bit to get air out of the lines so you can get pressure to build.

You can compress air...which is what is going on...but you can not compress a liquid.

DUB

Tewski

I thought about this. Question: to do this, do i fill the hard line at the fitting to the MC? Or should the reservoir automatically flow into the hard line, provided that the bleed screws are open? I ask because when the reservoir is full and no hard line is attached, i.e. open to the atmosphere, no fluid drains from the reservoir, even though i just bled it.

Thanks

Peterbuilt

If the front lines were replaced and filled up with fluid why wouldn't the M/C fill the rear lines?

Because the rear of the M/C is defective or is full of air.

MelWff

It sounds like your push rod is too long and is partially compressing the piston in the master preventing the fluid from coming out. Loosen the master and see if fluid starts to come out the open port.

Peterbuilt

That is a good possibility, M/C's come with a deep or shallow hole.
Did you get the correct one?

DUB

We were thinking along the same lines. If the wrong master cylinder was installed...that is a problem right there.

And to answer your question....YES...fluid should drip out of the rear line fitting are if the line is off and the cap is off the reservoir.

ALSO...knowing that you have the rear of the car up in the air and safely supported. Try to get it up high enough so your master cylinder is LEVEL.

DUB

Tewski

I did verify this prior to installation, I do have a deep hole type and the power booster has a long extension that slides into the hole.


I loosed both nuts and obtained about an additional .150 inches of clearance between the power booster and master cylinder, I obtained the same results as previously described when I pressed the brake pedal, just squirting air.

Thank you for confirming that it should be dripping with the line fitting removed, zero fluid is draining/dripping from the rear reservoir when it is removed. I currently have the car on 4 jack stands, I'm kind of paranoid about lifting the rear higher than the front, I'll see what I can do to get it level.

Thank you everyone for your input on this problem, I appreciate you taking the time to help me out.

MelWff

how could you have successfully bled the master and now you get air?
Do you have one of those kits where you screw in a plastic fitting to the master cylinder port with a rubber hose that runs back into the master reservoir? If so hook it up to the rear port and bleed the master using the brake pedal and see if the air goes away.

Pop Chevy

I like to use a Mityvac it pulls the fluid through the line. If not then gravity bleed it with ALL lines hooked up and tightened. Takes a while though , like open the bleeders and walk away.

Tewski

I do have one of those kits, and have attached it to the MC port and after a pump or two of the pedal it flows perfectly in the closed loop with no air bubbles, as soon as I remove it and attach the line I get noting, I loosen the fitting at the MC and its pumping air again. I just purchased another new MC and will pick it up and install it tonight and see what happens.

MelWff

why are you loosening the fitting at the master after completing the bleed?
after bleeding the master and reconnecting the lines have you opened the right rear caliper outer bleed screw and either gravity bled it or have someone depress the brake pedal as you open the bleed on the caliper, close the bleed, pump the pedal and hold it down, open the bleed and do a conventional brake caliper bleed.

DUB

Not being there to witness the 'bench bleeding' process and watching how the fluid reacts when the piston in the master cylinder is pushed in ONE INCH. The master cylinder can be at fault.

BUT...you are also 'saying' that you can hook up the bleeder to the master cylinder and get the rear line to flow CLEAN AIR FREE FLUID....which is telling me the master cylinder is GOOD.....BUT....that is also assuming that the correct master cylinder was installed in regards to the rear portion that is specific to a manual brakes or power brakes.

Keep in mind the rear brakes have a lot of line and it can take a lot of the manual bleed method to even get the brakes to feel like they are doing anything. The rear brakes ARE NOT going to bleed as fast as the front.

If your 'helper' who is lightly pumping the brake pedal and THEN holding down on the brake pedal....can feel a slight drop in the brake pedal EVERY TIME you crack the bleeder and let the air out and then close it. THEN they will bleed out.....BUT...keep an eye on your brake fluid level. And you might do two or three steps of the bleeding process before your helper feels something significant. They might tell you that the pedal has no resistance and it not doing anything....but do not give up.......It might start off that they can feel the brake pedal drop 1/32"...the it will being to get better and better.

And I can say that when I bench bleed a master cylinder at my vise...I can be dong it for about 1/2 hours to get it clean fluid with not bubbles...and the master cylinder is LEVEL while I am doing so....and then I still use my pressure bleeder.

DO not be surprised that IF you get it all bled out and they feel great....that the next day they go to the floor....and this can be caused by micro air bubbles that were clinging to the inside walls of your lines, hoses and calipers. IF this happens.....I shock the frame with a my air hammer and then bleed out the system again to get these small bubbles...that have now congregated into a much larger bubble out of the system.

DUB

mk's78

Hey there MelWff,

A ? 4 U.....When you gravity bleed the rears only can all of the bleed screws be open during the GB procedure????

mk's78

MelWff

I have never tried it that way, have always done one at a time since you normally start with the bleed screw furthest from the master.

sgt kohler

isn't the one on the right for power brakes and the left is manual?

DUB

I do not think that it is 100% that way all the time...because GM changed the power brake boosters design....I DO BELIEVE that the master cylinder on the RIGHT is for power brakes on a 1977-1982.

I DO know that a MANUAL master cylinder is like the one on the LEFT...FOR SURE. BUT...this same master cylinder can be used with an early power brake booster WITH the adjustable center pin. Right now I am having a total brain fart...but I 'think' I am right that there is a crossover where the same master cylinder can be used on manual and power...but do not slit my throat if I am wrong due to not being at work now where I could double check.

DUB

TOM B1

This is getting confusing so lets start over. Lets say the MC is on the car.
First, hook up the plastic MC bleeders with the hoses going back into the MC fluid and keep the hoses deep into the fluid all the time. Have someone pump the pedal "SLOW" and let up on the pedal SLOW. do this till all air is out. keep the hoses under the fluid and the reservoirs full.
Now hook up and tighten all lines. Here is where a set of self bleeders are real nice but if you have stock here is how to bleed them.
Start at the right rear. most vettes have 2 bleeders on each rear caliper.
Each caliper can hold about 4 oz of fluid. keep the MC Full.
attach a hose to one of the bleeders and put the hose into a cup or something. now have someone pump the pedal about 4 times then stop at the floor and hold it there. now open the bleeder, it will pop, now close it.
let up on the pedal slowly as you dont want to suck in air from the fluid getting to low. , check the fluid in MC and do it again, always stop with the pedal to the floor then open the bleeder as it will pop, then close it, let up on the pedal, check fluid again., do this to both bleeders on each caliper if yours has two. then go to the left rear. then right front, then left front. If you let the MC get to low on fluid and it sucks in air just one time then you need to start all over.
I had my wife pump the pedal and she got tired of it after the first caliper so get a good friend to help you.