69350rocket

My master cylinder went out a while ago. Replaced it with one from autozone. I have not had rear brakes since I have had the car. It was never really a big problem, except that now the pedal has almost no pressure going down, and does not return on its own. I have been trying to get brake fluid to the back brakes and cannot. I changed the wheel cylinders and still cant get fluid. I am using a vacuum bleeder to try and bleed them. Then I thought that maybe one of the lines was clogged, so I took it off at the master cylinder and sprayed brake cleaner in it. The brake cleaner came out both of the bleeder valves. So now I am confused, why can I not pull any fluid through the lines?

Allan R

This is for a 69 Cutlass? Did you bench bleed the MC before you installed it?

69350rocket

Yes. And yes I did, but Im starting to wonder if maybe I didnt do it enough?

69442C

Make sure the method of bench bleeding involves using a fitting with a small piece of hose that you run back up into the MC reservoir. When you don't have any more bubbles coming from the hose into the reservoir, the MC is correctly bled. Some instructions just tell you to place caps on the outlet and pump the cylinder until no bubbles come from the reservoir. This will usually work but since you are having problems, I would try the first method I mentioned. And old brake line with a section of hose will do the job if you have it.

The second thing you want to check is how the MC bolts to the car or the booster. When you place the MC in place, does it sit flush against the mating surface? If it doesn't, is is possible that the rod that fits into the MC is holding it out slightly? If this is the case, you will push the piston in the MC slightly inward when you tighten it down which will cause the piston seal to either go past the small fluid port in the bottom of the MC reservoir or it may just block it. When this happens, you can't pick up any fluid to push into the lines.

I don't like the sound of the pedal not returning. Do you have power brakes?

Lastly, are you using regular brake fluid or silicone?

Tony72Cutlass'S'

I was having HUGE problems last summer when i was first doing this, i literally had to submerge my entire master cylinder in brake fluid in order to get it bench bled properly.

Long story short, there's no harm in bench bleeding it one more time for good measure.

-Tony

69350rocket

Yes power brakes. And I just went out and bench bled it some more, only with it on the car and using the pedal. Some air came out at first, but then it got to where it will squirt fluid out. So, I hooked up the line and tried to bleed it again and nothing. I figured that would have done it. I am really lost now. I know the lines are fine and can get fluid through them, I just cant get fluid to come out from the master cylinder when Im trying to bleed the lines, which makes no sense because it would spray fluid out with the line off.

Oldsmaniac

If it were me I would disconnect the rear lines at the master cyl and blow compressed air thru them. Also replace the center rear rubber brake hose if it is old. The front ones too as they are often overlooked and get brittle. Also make sure bleeders are not clogged.

69350rocket

I already did that sort of, I used a can of brake cleaner and sprayed it into the line at the master cylinder, and it came out of both bleeders in the back. So, there shouldnt be any sort of block or anything in the lines. Also I just tried the old fashion two person bleed, and when the pedal was pressed I did not feel any air coming out of the bleeder, which makes me even further confused.

Allan R

You can't bench bleed it on the car. The angle of the reservoir doesn't allow proper bleeding. Take it off the car, clamp it horizonal in a vise and bench it that way. Chances are you have air trapped in part of the Master cylinder because of the angle. The venting holes are not at the end of the piston travel.

MDchanic

Is there a hold-off valve, and is it working?

- Eric

69350rocket

O, I was not aware of that. I will try that tomorrow. And no, no hold off valve.

69442C

The hold off valve, if present, is part of the front brake circuit so it would not have a role in the lack of fluid to the rear brakes. But since there is no valve, do you have 4 wheel drum brakes or front disc/rear drum? If the later, is this a factory system or a conversion?

You didn't address the question regarding how the MC fits against the booster without the nuts installed. Can you get the MC to sit flush against the booster or does it need the nuts to pull it down flush?

You mention when bleeding the MC that at some point it started to push fluid out of the MC rear port when you were bleeding it. How are you bleeding the MC? There should always be something coming out of the port, even it it's air, upon initial bleeding. Did you put a fitting and a hose on that port and run the hose up into the reservoir? Regular brake fluid or silicone?

Tony72Cutlass'S'

That DEFINITELY sounds like exactly what i had with my Master Cylinder.

Pull the MC off your car, and bench bleed it, if you wanna be 100% sure there's no air, dunk it in a bucket full of brake fluid and bench bleed it that way. Just maybe put gloves on :P

Cheers,

Tony

69350rocket

Four wheel drums. And yes I bled the master cylinder on the car and after that I took the tube out and pushed the pedal and fluid shot out everywhere. Bled it again to get rid of the air I had just sucked in, and put the line on, and nothing. I have tried bleeding it with a vacuum pump and the old fashion way. If I have someone press on the pedal with the bleeders open I get absolutely no air coming out of them. And with this master cylinder the piston sits flush with the front, and the other was deep down in it. I had to cut the pushrod down a lot to make it work. I was thinking this could be the problem, but if something was wrong with that then how would I be able to get fluid to shoot out of the port?

Oldsmaniac

And this could be related to the problem.... you got the wrong master cylinder and modified the rod to work. Maybe now the stroke is not right. Or maybe the rod is not in the correct hole on the pedal.

69442C

I assume you are referring to the rod that you cut down as the one that sits in the end of the power booster and fits into the MC? Don't like to hear things were made to work.

On the MC that you purchased, are the front and back reservoirs the same size or is one larger than the other?

Also, since you had to play with the rod that fits into the MC, I need you to clarify a point for me. I asked this before but you have not answered this. When you place the MC against the booster, does it sit flush against the booster? Or does it stick out a little requiring you to tighten the nuts to pull it down flush?

69350rocket

Tried both holes in the pedal already. Reservoirs are the same size. When I tightened down the bolts it did snug it up the rest of the way. But that was maybe a quarter of an inch. Thank you for the responses so far.

MDchanic

Are you saying that the new M/C can't be held flush against the booster, so you pulled it down with the nuts against spring pressure?

If so, there's your problem.
The piston HAS to sit all the way at the end of it's stroke fir the M/C to work, with a tiny bit of play (specified in the manual) between the piston and the pushrod.

- Eric

69442C

X2. I had a suspicion this was the issue which is why I kept asking this question. The MC should be able to sit flat against that booster without it being forced into that position by tightening the nuts. I think the spec is the rod should either just touch the end of the MC piston or there should be no more than .010" of clearance.

69350rocket

Alright thanks, Ill tell you guys if it works!