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MG MGB Technical - That old Brake Bleeding question

Hey, I know this question has been beaten to death but I'm grasping for straws. I have rebuilt the master cylinder on my '72 GT as well as all four wheel cylinders. I've read the archives and bled the system about a dozen times on each cylinder without luck. The pedal remains spongy after pumping up and goes completely to the floor when you let it rest for several minutes. I suspect the Master was beyond repairable limits and is bleeding back but I'm looking for some additional opinions before I shell out for a new or rebuilt unit. The Master did have some surface rust and I did use a hone on it and it appeared not to have any pits. Any help is appreciated.
Tony Mason

Did you bench bleed the MC before installing it?
william fox

Unfortunately not. I missed that one. Without removing the unit, can I bleed the unit in place?
Tony Mason

I think so. Disconnect the lines. If you have an old length of brake pipe (you'll need two of the fittings) cut off all but about an inch from either fitting and attach a small diameter plastic tube to it. Connect in place of the brake pipes, place the ends in a jar partly filed with brake fluid. Have someone pump the pedal while you keep a check on the reservior and keep it topped up.
william fox

Tony, an effective way to get the air out of the master is to just tickle the brake pedal right at the top for a few min. You can see the air bubble up through the reservoir. Just giggle the pedal and depress once then let the pedal flip back up. Letting it flip back dislodges some of those stuborn air bubbles.
Do that a couple of times and then do your normal bleed.
Kelly Combes

I really think the gunson's eezibleed is the answer
to this sort of problem. (Pressure bleeder
that works off tire pressure.)
It eliminates the need
to bench-bleed cylinders, and allows one-man (person,
sorry) operation.

Are you sure you don't have a leak somewhere?
Also, the rear cylinders are probably worth replacing,
not rebuilding. A slightly leaking rear cylinder
will let lots of air into the system, and the
pedal will feel like a problem with the MC.
Ronald

My car is doing the EXACT same thing right now, except I rebuilt my front suspension over the weekend and cleaned the whole assembly with degreaser that states "keep away from rubber" well, I suspect the rubber seals in my brake calipers went south of the border, and they're letting air get sucked in, without letting brake fluid out. Don't ask me how, but this is all I can come up with after 2 days of bleeding all 4 wheels without having touched any other part of the brake system during the process.

Justin
Justin

Thanks guys. I'll keep checking. It's just frustrating as this is the last problem of a 2 year rebuild and it's keeping me off the streets!
Tony Mason

I've heard good things about the mityvac brake bleeder. Has anyone tried it?
Greg Bowman

Tony,

If you've bled the system several times, then whatever bubbles you'd have removed by bench bleeding are gone by now. You can simulate bench bleeding by removing the reservoir cap and use a rod (large blunt phillips) to push the cylinder a tiny bit in and out. If you'd done this before bleeding, you'd have found little bubbles rising from the port on the bottom.

You could try adjusting your rear brakes completely tight. This gives your pedal something to push against when you're bleeding those cylinders. If the adjustment is too loose, then the system won't develop as much pressure.

If you suspect your master cylinder, then: Sit in the car and hold the brake as if you're on a steep hill. Not crushing, but not so gentle either. After several minutes does the pedal start moving slowly to the floor? If so, then your master cylinder seals are not holding pressure.

You might also check all of your connections and lines for leaks.

Hope this helps,


Matt Kulka

If the pedal sinks then the MC is shot, bench-bleeding or no. There are as many methods of bleeding as voices to say them - almost. Mine is a Gunsons first, then my beautiful assistant standing on the pedal while I rapidly open and close each caliper nipple. This second step always gets out air that the Gunsons doesn't.
Paul Hunt

I think my stupidity is showing here, but I put my calipers on the wrong sides, and the bleed screws are on the bottom of the calipers as opposed to the top. When I get home from work tonight I'll humbly swap the calipers to their respective sides, bleed, and drive for the first time since Friday.

Maybe this is applicable to others?
Justin
Justin

Justtin:

I feel for you bor.

I just finished replacing my new calipers and S/S brake lines this w/e. Took me two hours to find out that at the angle the car was in (the front end was a bit in the air), I wasn't getting any brake fluid to the front of the resovoir therefor only pumping air to the rears.

duh!

Good luck
Luis

Luis,

I spent 3 solid days cleaning, scrubbing, reaming and meticulously preparing all the new parts for the front suspension, and I never gave two thoughts to the calipers. I took them off, cleaned them, and set them back in the pile, only to be thought about again when I picked one up and stuck it back on the car. DOH!!!

So how's Miami these days? I haven't been there since March of 2001 when I was last dating a girl there.... mmmm Cuban girls..... *big stupid grin* =)

Justin
Justin

Don't forget to use a bottle and hose when bleeding , otherwise you're just wasting time .

I use a Mity-Vac occasionally , it works out pretty well .
-Nate
Nate

Now I'm feeling a little daft! Justin, tell me switching those calipers right side up made the difference! Mine have the bleed screws on the bottom too!
Tony Mason

If the bleed screws are on the bottom, then you
can't get rid of the air bubbles that have floated
to the top of the calipers. (This is a common
mistake with clutch slave cylinders too.)
Ronald

Justin:

Miami's fine except for the 109 Degree heat and the terrible traffic jams. Loks like we're gonna get some rain from TS Isidore this w/e.

Luis

(Top down since.........Dec?) Yes, I do put up my tonneau cove when it rains.

Cuban girls = "Can you put your top up, my hair may get messed up". Great coffee, tho.
Luis

Justin,
Thanks a lot! Turned those puppies around and bled them once more and I've got brakes! It's great that you are willing to share some of the horror stories that we all do once in a while! We all learn from our mistakes. It's even better when we share them so we all can then avoid doing the same thing. Thanks again to everyone else that participated. I'll be sure to keep in touch and hopefully repay the favor.
Tony Mason

No prob, I'm always happy to share my bouts of stupidity with others. =) I think I got my brakes fixed too and MAN was it bizarre. While bleeding, the air bubble forming in the resevoir formed at the front of it, as opposed to the top, so the piston for the rear brakes was suckin air down into the system as opposed to juice, and the fronts always had juice.... go figure....

Justin
Justin

This thread was discussed between 17/09/2002 and 19/09/2002

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.