I just got a new brake master cylinder and am trying to bench bleed it. The problem I'm having is that brake fluid keeps filling up one reservoir and emptying the other. I checked and its level in the vice. I see no leaks. What is going on?? All the air bled out fine, but I'd hate to put it on the car and have this happen as I drive.

hm... it's been several years since I bench bled a master cylinder. But something in my head is telling me that the piston might have been installed backwards???

I've never had to bleed one so I'm no help. Here are a couple of short articles on it - any good info in there?

http://www.superchevy.com/technical/c ... s/0509sc_bench/index.html

http://autorepair.about.com/od/fixityourself/ss/MC_bench-bleed.htm

Matthew

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Thanks but the links dont really help me. I have two reservoirs on my '90. I'm thinking it could have something to do with the proportioning valve.

Yeah I kinda figured that after the fact. Did you get it sorted out?

Matthew

I re-seated the proportioning valve and bleed the brakes. It seems to have fixed the problem. Thanks for the help!

Don't put too much emphasis on that technique. If you would like to bench bleed...a better technique would to be to mount the MC and use the brake pedal to run fluid through it.

OK then you have no air in the MC. Once you take the tubes off to install the actual lines....what happens?? More air.

So, I'm not one that cares too much about the BB thing.

Most people get into trouble with the bleeding process by not pushing enough fluid through the Right Rear caliper....you have to make sure that the bubble I just described in your brake line travels all the way from your MC to the RR caliper.

Once you get into this situation, many 'ideas' crop up about vacuum thingamajigs and self bleeding valves etc.

if you get the correct ID clear tube and a coke bottle, hook the tube to the bleeder valve and run it down into fluid in the coke bottle......you'll see the bubbles coming out. When you crack the bleeder, don't open it all the way....allow it to be under pressure, like your thumb over the water hose. If you do it this way, you'll hear the air bubbles as they make their way out of the Bleeder Valve.

You still need a friend to faithfully listen to "pump", "Hold" and "release" commands from you...but this is a 10-15 minute process if done correctly.

And remember, as the fluid comes out, you'll be watching for color change. When you are getting close to the bubbles...the fluid will be almost clear/new. When it is clear you know that you've pumped enough fluid through and that the bubble must be close.

As many threads that get started on this subject, and the fact that I've never really had issues...the only thing I can think of that that people just don't pump enough fluid OR that they nor their friend understand the "pump, hold, release" commands properly.

I'll state this last thing about your friend. They don't see what you are doing...they are bored out of their mind as you drone on. BUT YOU are practicing a technique. When you state for them to "push"...you are cracking that valve open as then push...then you allow a bit of fluid out but you shut the valve prior to their foot reaching the floor. So the Valve is only cracked open for a second. This maintains a positive fluid flow allowing no back streaming into the caliper. Your friend must keep their foot pressure on the pedal until you state "release"...this in case your wrench slipped off the Bleeder.

The Right rear is the hardest to do. That Left Front one should be last and your friend will be surprised as you rap it up with only 3-4 pumps.

But that Right Rear has to be the issue why so many have a spongy pedal afterwords, which leads to so many strange techniques like "bench bleeding" being developed....

I think you are over complicating the process, John.

I last did a bench bleed about a year ago for bastet's car. The idea behind it is simple - so much air builds up within the master cylinder itself, that if you try to on car bleed it, you will be there forever undoing it.

I didn't have any issues with the master on car, either.

I think I nailed it when I felt the master wasn't assembled correctly. And no matter how he bled the system, that problem would have existed regardless. As it is, bench bleeding found it before putting it on the car.

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Yup, you hit the nail on the head! Thats why I keep coming here and trusting the advice I get!

Yeah, brake bleeding is simple. I just installed a brake bias spring this morning. Didn't even remove the master cylinder from the car.

All I do is open 1 bleeder, tell my 10 year old son to push the pedal to the floor and hold it there, then tighten the bleeder and tell him to let off. I don't do the pumping the pedal in between, or the opening of the bleeder after pedal pressure is applied. Just simply open bleeder, floor brake, close bleeder, release brake. After about the 3rd cycle I get a bunch of air, after about the 6th, it's all fluid. I do about 10 cycles just to clear out all the old fluid. Very simple and always works like a charm.

Quote:I'm trying to over simplify it.

I don't think the Pump, Release, step is explained well enough and people aren't pumping enough through or getting back wash by letting the pedal loose while the Bleeder is still open....

I do know this...and I'd be the first to listen as I make plenty of mistakes...but the brake bleed process always leads to these types of chats......It almost takes me longer to type it out than it would to bleed one.

So there has to be a disconnect somewhere between what I am doing versus what others are doing. Even CentralCoaster has a difference of opinion than I do on bleeding....yet I can bleed one in 20 minutes...and is seems the last three fixer up cars I have gotten all needed Master Cylinders....

And getting a miss-assembled MC is different altogether.

I agree with that contention. I read your post last night and all the words started to blur... my bad... so I missed your point.

I have always been firm with whomever was doing the pumping... cause you can see when someone releases the pedal too soon.

Yea, well... I just asked the killer question - where to get a good master...