Bubbles in Master

[N9nefingers]

Welp I've searched and found some threads on this but no one seems to have posted the solution. I re-did my brake system in the 70 C10. Went with disk up front, left the drums in the rear. New booster, master, prop valve. I bled the master on the bench and bleed the brakes but the pedal sucked. I keep bleeding but nothing really improved so I figured I'd start back at the beginning. This time I have the master still on the truck, I bent new lines from the prop valve outlets to the reservoirs and started to bleed. I get no bubbles from the lines themselves but I get several small bubbles from the rear reservoir, the hole that the fluid leaves the reservoir itself. This seems like it should not happen. But no matter how many times I pump the brake I get several small bubbles coming from that hole. Ideas?

[Big Kev-O]

Sounds like you got air inside the master after your bench bleed. You may have to pump the brakes/master cylinder piston over 50 times before you stop seeing bubbles. It can take a long time to get all the air out of the system especially if you lines are empty. If the bubbling does not stop with the master connected to the prop valve you may need to disconnect the master from the prop and bleed it again using tubes or plugs.

The last two masters I changed came with plugs in the outlets. I made sure the plugs were snug, filled with fluid and pumped until the bubbles were gone. I have seen warnings not to bottom out the piston when bench bleeding because it can damage the internal seals.

IF that does not solve the issue one of you most likely have leak in a brake line or master cylinder allowing air to enter the system.

[LockDoc]

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On the rear drum brakes it all begins with adjustment of the brake shoes.

On the drum brakes, one thing I found on mine is when you put everything back together and adjust the brake shoes you can’t just adjust them out until they start rubbing and quit. You have to adjust them out as far as they will go and you cannot turn the wheel or drum at all. This centers the whole assembly in the drum. (I even tap around the drum with a hammer as I tighten them to help them center themselves) Then you back them off until the wheel or drum just turns, and stop. If you only adjust them out until they start rubbing and quit, the first time you press on the brake pedal the brake shoe assemblies will center themselves and you will have too much clearance between the shoes and the brake drum. If there is too much travel in the shoes the pedal will go down too far before the shoes contact the drums. I also put three lug nuts on backwards to hold the drum tight against the axle on the rear before I adjust them.

I know it is a pain in the butt to try and hold the self adjusters off of the adjuster wheel while you back them off, but this is the only way I could get my drum brake shoes adjusted correctly. This may help with your sucky pedal....

LockDoc

[N9nefingers]

I was thinking air in the lines or not properly seated/adjusted drums in the rear but figured I should start back at zero and "bench bleed" the master cylinder again. So I could rule it out and move on down the line so to speak. The issue I'm having now is those strange little bubbles. I'm probably not explaining it well...I left the lines from the master to the prop valve, then ran new short lines from the prop back into the top of the master reservoir. The air bubbles aren't coming from the lines at all. They are very small bubbles trickling out of the hole at the bottom of the rear reservoir of the master. I get good fluid movement from the lines with no bubbles. Am I worried about nothing at all? Should I install the lines to the brakes and bleed those and see what happens?

[Andy4639]

It could be a bad master cylinder also.

[Wrenchbender Ret]

If you are getting bubbles out of the ports in the master cyl. you are on the right track. Keep working the master cyl. till they stop. You don't have to push it all the way. Just push the pedal a little & let it back. Once you get all the air out of the MC the rest will bleed easy.
George

[N9nefingers]

I was worried I may have a bad master out of the box. I’ll keep pumping away and see if I can get the bubbles gone. Thanks everyone. I’ll post back the results.

[N9nefingers]

Just a quick thing to say thanks for everyone's help...turns out I don't think I really properly bleed out the new master. I was getting good, strong fluid flow, without bubbles back into the master while bench bleeding but I think the bubbles coming up in the back port was causing my problems. What I did was pump away then leave the masters lid off for while I was at work, 24 hour shifts. Came back to no more bubbles coming up. Bleed some more just to be sure then installed the brake lines as normal and bleed each wheel. All good now. Thanks again to everyone who chimed in and helped.

[LockDoc]

Originally Posted by N9nefingers

Just a quick thing to say thanks for everyone's help...turns out I don't think I really properly bleed out the new master. I was getting good, strong fluid flow, without bubbles back into the master while bench bleeding but I think the bubbles coming up in the back port was causing my problems. What I did was pump away then leave the masters lid off for while I was at work, 24 hour shifts. Came back to no more bubbles coming up. Bleed some more just to be sure then installed the brake lines as normal and bleed each wheel. All good now. Thanks again to everyone who chimed in and helped.

Glad you figured it out. Sometimes that stuff can be a head scratcher for sure.

LockDoc

[697498chevy]

Where do you fellas get a master cylinder bleeder kit ? Just came from NAPA and all they had was metric ? THANKS !!

[Steeveedee]

Originally Posted by 697498chevy

Where do you fellas get a master cylinder bleeder kit ? Just came from NAPA and all they had was metric ? THANKS !!

You'll probably have to buy adapter fittings, and the NAPA store probably has them.

[N9nefingers]

I was already making brake lines for the truck. I got the fittings from speedway or stopshop, can’t remember. The way that worked for me was to mount the master on the truck then bend lines from the ports back into the reservoirs. Worked great. No kit needed.

[697498chevy]

OK , Thanks !!

[697498chevy]

My original master has bleeders above the brake line fittings . New one doesn't . Seems kinda strange . Oh well , I'll figure something out . Thanks !

[697498chevy]

Went back to NAPA with Dorman bleeder kit part number (SAE) and after some looking they found just what I needed . NAPA #700-1132 , $8 . Bench bled the new master and have it installed . Now I gotta bleed the wheel cylinders . Thanks !!

[RichardJ]

>>What I did was pump away then leave the masters lid off for while I was at work, 24 hour shifts.<<

DO NOT leave the top off the MC or the cap off a bottle of brake fluid. Brake fluid is Hygroscopic and will absorb moisture out of the air.

Air will always rise to the top in any liquid if there is a direct path. If a MC is mounted horizontally, any trapped air will find its own way out when the pistons move to the rear uncovering the holes.