I recently replaced the rear brakes (shoes, springs, wheel cylinder) and tried bleeding the brakes. After spending a good deal of time bleeding the brakes, thinking I was making progress, I would get to the point where I had no more air bubbles but once I would go back to the other wheel, the air bubbles would return. After a close inspection I noticed a small amount of fluid coming from the back of the Master Cylinder. So I figured that's where I kept getting air introduced into the system.

Replacement of the MC complete. now I go to bleed the brakes again but now I get literally no fluid coming out. I chocked it up to a good deal of air in the lines. After spending several hours of the old fashioned, have a friend pump the brakes then crack the bleeder with no progress. Sometimes I wold get a small amount of fluid and sometimes nothing at all. This was the case at all 4 corners.

I figured I would change up my bleeding method. I tried gravity bleeding. I tried by one wheel at a time for a few hours but not a drop of fluid. SO I went with all 4 corners, still not a drop.

So i decided to up my game. I picked up a hand vacuum pump.
Well no luck either. Spent about 6 hours overall trying to pull the air out the lines but that's all I would get was air. Occasionally I would pull a small amount of fluid but mostly it was just air.

So what on earth am I doing wrong. I have bled brakes many times before, even after replacing entire lines but never have I had this much problem. I used the vacuum pump to pull air out of the MC but maybe that's not as good as running hoses into each reservoir and bench bleeding. I'm not sure.

What concerns me is the lack of any fluid even dripping from the lines going to the proportioning valve (which I removed for troubleshooting purpose).

Could It be a bad MC or do I need to bleed it better maybe?

Sorry for the long post just thought a little back story might be needed.

Vehicle with problems is my 1984 Bronco XLT 5.0

 

If it were me, I'd pull the master cylinder, chuck it into the vise, and bench bleed it using the tubes back into the reservoirs.

I had a new MC for a Merc once that even trying to bench bleed it was tough. Just kept after it, tried stroking it slower, which is a trick pushing against a spring with a big punch. Finally got it to squirt reliably back into the reservoirs, and just kept it up till no more bubbles. Took a while!

When installed, no problems.

Trying to bench bleed it will also tell you after a while if you have a dud MC or not.

 

Just did the bench bleed. There was a small amount of air but not much.

Well I am still having the same problem. With the vacuum pump hooked up, soon as I crack the bleeder I get nothing but air. No fluid at all. This is from all 4 corners. I check every line where it connects even to the proportioning valve and there are no leaks.

I am getting frustrated at how simple a task this should have been but is now becoming something so much more. Any advice is greatly appreciated. Thank you,

 

A couple of thoughts...

I've never had good luck bleeding brakes by applying vacuum through a hose to the bleeder screws. Seems I always got air bubbles, never solid fluid. Air would sneak in at the bleeder screw's threads. And if I turned the bleeder screw in a little further to try to get a better seal there, then the screw port was too far closed for fluid.

MC Compensation Ports - When no pedal effort is applied, the MC's compensation ports are supposed to be Open. They are slots in the MC bore that go up to the reservoir. Usually, just pulling a brake line off at a wheel will result in a drip drip drip of brake fluid out of the open line connection. Gravity pulls out the fluid, and the MC reservoir supplies new fluid into the MC bore via the comp. ports. The comp ports also make sure that there is no residual pressure in the braking system with foot off of the pedal, as fluid flows back from master bore to reservoir.

The comp ports close when there is some movement of the brake pedal forward. If a brake booster has its output push rod screwed out too far (on adjustable ones), or the brake pedal assembly is not returning fully to the released position (pushing the booster push rod out via mechanics of booster), the MC piston might be being pressed forward some, when it should not be (therefore closing the compensation ports when they should be Open). If they are closed, ya ain't never gonna get fluid at the wheels.
EASY TEST - Loosen both MC to Booster mounting nuts a bunch of turns, and pull the whole MC with everything attached forward like at least 1/4-3/8". This will make sure that the booster push rod (for whatever reason) is not pressing the MC's piston forward. Then try pulling vac at the wheels again.

 

Still no luck. It seems as though fluid is not transferring to the wheels. When I use the vacuum pump I can pull out very small amounts of fluid from the MC but not much. It doesn't get better the more I do it. I placed grease on the threads of the bleeder to ensure I wasn't getting air past. but still same problem.

I can't even gravity bleed or get any fluid out of the MC via gravity which makes no sense. I can press the MC piston and get fluid to shoot out when bench bleeding. I can use a syringe to push fluid through the lines and it comes out which ever bleeder screw is open. I just don't understand. Could it be a bad MC somehow that isn't allowing fluid to fill the brake lines?

Ive tried regular bleeding, vacuum bleeding, and gravity and non of them work. I am in need of some good advice.

 

Maybe the bleeders are obstructed?, can you get new ones and try again?.

 

The bleeders are new and I have made sure they are clear. I can take them completely out and nothing. I have even disconnected the top lines at the proportioning valve?? and nothing comes out of them. I can't help but think something is wrong with the MC not allowing fluid to travel down the lines

 

So I am almost certain it is the compensation ports that are blocked somehow as no fluid drips from the outlet ports. I understand its a new MC but could there still be an internal fault such as a seal blocking the port or debris clogging them? is there a way to clean them out such as a small needle?

 

Is this with the MC ON or OFF of the truck?

If it is ON the truck, did you try pulling the MC forward so the booster push rod is not for-sure pressing in hard on the master piston?

OFF the truck - using the bench-bleeding kit of two plastic adapters and two tubes, will it still bench-bleed? Or is it no matter what you do, no fluid will come out of the tubes? If it will no longer pump fluid using the bench-bleeding method, then something is indeed wrong with the MC.

I guess it is possible, but not probable, that old decayed rubber bits from internally-decaying brake hoses could have moved into the MC. At the bottom of each MC reservoir there is a little hole or slot. That is the passage down into the hole/slot in the MC bore that makes the compensation ports.
If one wanted to see or squirt solvent through the 2 comp. ports, then the guts of the MC bore need to come out.

That can be done by chucking a mounting ear into a vise, just like bench bleeding.
Then, while pushing in hard on the MC piston, remove the internal snap ring.
Then SLOWLY easy up on the pushing.
Take out parts carefully, lay them in order (no mistakes!) on a clean cloth, also noting direction of rubber cups.
It all has to go back together exactly the same way.
Look at MC repair kits on rockauto.com to see what all the parts look like.

Then can look in and see the ports in the bore wall. Can blast away with solvent.
Lube the bore with brake fluid when putting it all back together, to avoid damaging the rubber cups and seals.
This voids the warranty, of course.

MC rebuild kits are available that have all the parts and instructions like above. But usually not worthwhile on a cast-iron MC, as they often rust beyond where the piston cups usually ride in normal brake application, and the rust will tear-up new cups. Even honing them out a bit usually won't work, as the rust pits are often too deep.

Just in case, remember that when bleeding on the truck, the MC lid with gasket is just laid very loosely on the MC, not bailed down. The lid vent port that allows air to go in on top of the rubber bellows is small, and might not allow air in fast enough for the bleeding operation. I often set the lid on a little bit skewed sideways to let air in, just didn't want any geyser from pedal operation to spew brake fluid all over the fender paint!

I still wonder about the booster push rod/pedal assembly not going to the complete at-rest position. That's why I have mentioned pulling the whole MC forward.

 

I did move the MC forward about 1/2 inch or so to take any pressure off the plunger and still no drip from the MC. This is even the case when the MC is not even on the truck.I am able to bleed to MC the bench bleed way. When I depress the MC I get fluid squirting out but once the plunger is back into neutral position there is no fluid drip as there should be coming from the MC outlet ports.

 

I hace decided to return the MC. It seems to be defective in some way with the compensation ports. I am still boggled as to what the problem is. I can use a small syringe and push fluid through the port and it will come out of the outlet of the respective reservoir

I also noticed another problem while doing yet another bench bleed. The plunger I would push on would not fully retract or depress. I would apply pressure and once I slowly let go it would stop with about 1/2 in until fully retracted. I then would have to use the syringe, fill it with fluid and inject it into either tube that connected to the outlet port adapters. I really don't know what that is all about.

If there is anyone that can give me a few more ideas, that would be great. I will find out soon if another MC fixes all these problems or if there's something else I am doing wrong.

 

Well it turned out to be a bad master cylinder. I am not certain what the problem was but I returned it and the new one worked much different right away during bench bleed. While operating the plunger I would actually get air bubbles coming from the inlet ports, something the other MC didn't do. I then was able to bleed the entire system and have brakes again.

Times like this I sure wish that limited-lifetime warranty would cover time put into the project because I spent nearly a week troubleshooting only to leave me with returning the MC.

Thank you everyone for your assistance getting through this frustrating problem

 

Glad you were able to resolve it!

Was it a rebuilt MC? If so, what was the company that did the rebuild?

 

It was a Fenco Reman/Master Cylinder from Autozone.