Brake Master Cylinder Bleeding

[JEFFTATE]

Please help !

I'm having trouble bleeding the new stock replacement brake master cylinder on my '69 Camaro.
I got one from Year One a while back.
It's the kind with bleeder screws on the master cylinder itself.
It's been sitting on a shelf for a while.
It bench bleeds fine, but when I install it on the car , I continue to get air from the bleeder fitting at the calipers.
(I've got a clear bleeder hose in a bottle of fluid.)
All the fittings are tight.
There's no leaks.
The reservoirs are full.
I've put 2 whole big bottles of brake fluid through , and I continue to get air out of the front lines.
I even put Teflon tape on the bleeder screw threads to keep from sucking air back past the threads.
I've tried a vacuum bleeder also. Same result.

I thought that maybe the master cylinder had a bad seal inside it.
So I got another one from a parts store because I didn't have time to wait for a replacement from Year One.
The one from the parts store does the same thing.
I'm trying to get the car ready to go to the Motor State Challenge in 4 days !!

The directions that came in the new master cylinder suggested that
it may be necessary to lower the front of the car to get the master cylinder level. To help prevent trapped air inside it.
But why ???
I bench bled it.
There's no air in it.
Where's all the air coming from ???
Could it be sucking air in from somewhere ??
Like the rear of the master cylinder ???

I've bled and flushed a hundred cars' brake systems as part of routine maintenance. ( I used to be a tech )
And once I'd gotten the air out of the master cylinder , the remaining air in the lines just pushed right through and came out at the caliper bleeder screws .
So what's wrong here ???

Why is a '69 Camaro different from say .....a Volvo , or a BMW ???

Please help !!
I've got a deadline !!
I've gotta' get this car ready to go to Michigan in 4 days.
My car is on jackstands and my hands are dirty !!!

[derekf]

Seems like I saw something at one point about a bleeder that puts a vacuum on the MC reservior - that way the air is free to rise to the top like it wants too anyway. One of those might help, if I knew the details on what it was.

Given that you've tried two different MCs I'm going to go ahead and hazard a guess that it's not the MC.

Are you getting air consistently at all four wheels?

[JEFFTATE]

I'm getting no air at the rear wheels , just the front.
I've been surfing the net .
Found some articles saying not to push the master cylinder all the way down when bleeding.
I know this , but why ????
What would that affect ??
Where does the air come from ???
Also , my car has one of the original type proportioning valve / metering block / switches mounted under the master cylinder.
I wonder if this has something to do with it ??
On some models , you have to push in a rubber button on this switch when bleeding..

I'm lost .........

[David Pozzi]

Jeff,
Do you get air at the calipers too?
could it be sucking air past where the bleeder hose goes over the bleeder?

Run the hose upward 4 or 5 inches into a container and keep the end of the hose even with the top of the container, this will prevent siphoning and see if you still see bubbles in the line.

Articles say don't depress the brake pedal fully because it may push the MC piston past it's normal travel area and into a cruddy part of the MC bore which may damage the MC piston and cause a failure of the MC.

You can do a quick reverse bleed by putting a c clamp over the caliper and pushing the piston back a bit. This pushes fluid into the master cyl and reservoir pushing air out.

The metering valve on the front line can leak air into the front. If you suspect it, remove it and replace the line with a short bundy flex piece. You won't notice it gone.
David

[JEFFTATE]

Thanks David,

Yes , I'm getting air at the calipers.
But the bleeder hose is very tight where it pushes onto the bleeder.
The master cylinder is new ( rebuilt).
How much crud could I be getting into by pushing it too far ???
Am I not supposed to push it all the way ??
Even if it's new ???
Is there a chance that it is folding the lip of the seal backwards when I push it too far ???
How could this keep introducing air into the system ??
I could see where that might cause a pressure problem or failure problem, but how would that cause air to get into the system ???
Anyway , I haven't been pushing it all the way down.
I was just wondering why this could cause a problem.

I wasn't getting air into the system before I replaced the m/c.
I just replaced the m/c because it was seeping down the booster.
The brakes were working fine and I wasn't losing any fluid.
It was just damp on the front of the booster.

If the valve is leaking , are you saying to remove the valve totally and put a line in its place ???
Is the valve not necessary ???

I took the master cylinder off ( again) and bench bled it ( again ).
I'm gonna' reinstall it and bleed the system using the two person method ( making sure not to push the pedal all the way down )
Maybe it's just sucking air back up out of the lines , and into the master cylinder bores , when I first start to bleed the system. And it's getting trapped there and doesn't want to come out.
I've been trying to bleed it using the one person method with the bleeder open and the line into a bottle.
It just pulls the air right back into the master cylinder when I let off the pedal.
I never can get the air to push all the way through.
The air just gets pulled back up the line.

I think I'm starting off with a ( bench bled ) master cylinder that has no air in it , but the first time I depress the pedal and then let it back up , I'm pulling air from the lines back up into the m/c.
Once the air gets back in the m/c bores , it won't push back through ??????
Could this be right ??

Maybe if I do the two person method ( with closing off the bleeder each stroke ) , I won't pull the air back up into the m/c ???

[David Pozzi]

I've never had any problems after bleeding full stroke. I think the master cyls with the sealed lids are way less prone to having crud inside to cause problems.
No problem going full stroke with any kind of decent condition MC, especially a new one.

If your metering valve was bad, that would allow air in the system. Also if your lines are not sealing or seated right. Try pumping the pedal and holding down on it for a few minutes, a leak may show up if you pressurize the system.

The metering valve is only of benefit on super slippery surfaces like ice and with auto trans and the choke is on high idle with very light pressure on the brake pedal. Once line pressure is above 40 psi, the valve does nothing anyway.

Here's a quote:
"Metering Valve: This valve, used only on disc/drum systems, is variously called a “pressure regulator valve”, “metering valve”, or “hold-off valve”, depending on which GM publication you have.

The round metering valve is inserted in the system between the front master cylinder outlet (which feeds the front discs) and the front chamber of the distribution block. Its function is to hold off any fluid flow to the front disc calipers until the valve sees 30-40 psi of pressure; this ensures that the rear drum shoes have expanded into contact with the rear drums before the front disc calipers begin clamping on the rotors, balancing initial brake application to avoid the disconcerting front end “dive” associated with “front brakes first” under light brake application.

It also prevents a disconcerting situation which can occur (without the valve) on ice or snow under light brake application (like at a stop sign or red light) where the front wheels are stopped by the disc brakes, but the drum brakes on the rear wheels aren’t yet fully engaged and the rear wheels continue to rotate under idle torque, causing the rear of the car to move sideways."

Bleed it with two people, depress the pedal and close the bleeder, lift pedal, repeat.

[JEFFTATE]

Ok ,
I'll look for leaks,
Remove the valve if necessary.
And do the two person method.

Thanks David !!

[megaladon6]

back when i was a tech i had a couple of cars do that. what worked for me was, once the M/C is installed and and bled, i'd gravity bleed the system.

[JEFFTATE]

Ok , I figured it out !!!!!
The hold - off / metering / pressure regulator valve was the culprit.

Here's what was happening.
When I bench bled the master , it was ok.
No air was in it at all.
When I installed it on the car and began to bleed the system , the rear brakes bled fine.
All the air came out of the rear lines perfectly.
BUT , when I moved to the front lines , and began to bleed them, I got fluid out of the lines for the first couple of pumps and then air,air,air !!
It would not stop.
On the down stroke , fluid was being pushed into the catch bottle.
Then , when I closed the bleeder and let the pedal up , air was being sucked back into the front line through that valve !!!
That's why I kept getting air in the front lines.
And no air in the back lines.
I simply removed the un-needed valve and made a hard line to connect the master cylinder to the distribution block.
Then , the front brakes bled in a matter of minutes.

JUST LIKE DAVID POZZI ADVISED !!!!

Thanks David !!