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MGB MGB Brake Bleeding

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glemon

glemon

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I am flumoxed with bleeding the brakes on my 1979 MGB LE--I got the car a couple weeks ago, pedal was very spongy and low, tried to bleed the brakes a few days ago, bleed screw (actually the rear wheel cylinder) was stripped, couldn't tighten it down to seal good, don't mess with cheap fixes on brakes, so got a new RWC.

Installed today, acting odd, hard to bleed, if I open it enough to really let the fluid out it gets awfully bubbly, read about bleeding these, suggests a short opening of the bleed valve just as the pedel goes down. Tried this, seemed to work better as far as air coming out the hose, but still kind of bubbly, but I ran and ran fluid through, it has to have cycled through the system.


Anyway I bled and bled the right rear, then moved on to the other wheels, they all seemed to come out pretty normal-clean, bubble free.

Anyway, the more I bleed the worse they seem to get, now they pretty much go all the way to the floor no pumping up either, they didn't really seem to pump up before, which make me think it might not even be an air issue.

I am thinking there is some issue with the right rear or the master cylinder, so first question, which fitting should be used to bleed the master cylinder?

Other thing, I have worked on lots of british car brake systems, but always older, this is the first with boosted brakse, don't know if the booster could be causing the problem, I don't really know how they work.

Last thing, my bleeding assistant (wife) is running out of patience--so I sure hope the next thing works.

I have a Haynes manual coming but not yet here.
 
First things first... other than the spongy pedal, were the brakes working properly before bleeding? If you hold the pedal down, does it hold position or slowly fall to the floor?

Try pumping the brake pedal to build a bit of pressure. On the last pump, hold it down hard. While holding down, loosen the bleed screw and tighten again before the pedal is let back up. The pressure may force more air out.

There is no "bleed screw" on the master cylinder. It can only be bled by disconnecting it from the rest of the system. This is best done on a workbench. Considering the effort involved in removing and replacing the brake master, I'd try something else first. If you do remove the master, you may as well rebuild it while it's out. A rebuild kit is inexpensive and not difficult to install.
 
I hav no pedal now, but before it was soft, but didn't seem to slowly go down further if I held it down, nor did it seem to pump up.

No sign of leaking or losing fluid, brakes worked in a normal manner, but don't think I had enough to lock them up.

I know there is no bleeder on the master cylinder, there are three lines coming out, I have successfully bled master cylinders by slightly loosening the pipe fitting on the pressurized pipe to allow air and fluid out. I know I owuld need to bleed the whole system after this.

How would you bleed a master cylinder on the bench then install it without leaking air and fluid? never tried that.

According to the previous owner the system is gone through and at least rubber bits are new, which they appear to be.
 
Bench bleeding is only to get the air out of the master cylinder itself. It rids the bore of any trapped air bubbles, making regular bleeding easier. You then pump fluid through the system with the brake pedal, being careful to never let the reservoir get too low and suck up air. The problem with bleeding the master while in the car is the mess. If you're using DOT 3/4 fluid, it will eat away any paint in comes into contact with and you will then have a rust problem. I suppose if you're careful you can get away with it.

I'm betting you still have air in the system but it's always possible the rebuild kit was the wrong type, the rubber bits were damaged during installation or installed incorrectly, or that the booster is bad. If the booster is leaking, it will be full of fluid and your brake fluid level in the master's reservoir will drop as you use the brakes.
 
Bentley recommends that you empty the MC by bleeding it through one front caliper and one rear brake cylinder before removing the MC from the car for rebuild.
 
"<span style="font-style: italic">Try pumping the brake pedal to build a bit of pressure. On the last pump, hold it down hard. While holding down, loosen the bleed screw and tighten again before the pedal is let back up. The pressure may force more air out."</span>

My friends who work on American cars have used that method well over the years but I have never used it on the LBCs I've worked on. I suppose it's worth a try. However, before you do... I have a question about the rear brakes on late model MGBs. Do they have a rear brake proportioning valve? Some of the LBCs I've worked on have a valve that shuts off (limits) pressure to the rear wheels during a hard stop. The pump-pump-pump-hold-open method of bleeding brakes won't work where those are fitted as the process closes the proportioning valve.

Though this is not recommended in any manual, you could consider wrapping the threads of the bleed nipple with Teflon tape prior to brake bleeding. Use the pump-hold-open-close-release method of bleeding. The Teflon tape will minimize (or prevent) air bubbles from getting sucked back around the threads and into the system. If the connection to the wheel cylinder is tight there should be no other sources for air to get in. Before anyone says something alarming about Teflon tape on the threads... keep in mind that the nipple seal is not made on the threads, it's made on the tapered tip. When bleeding is carried out as I describe above there is no chance for threads of Teflon to be "sucked" upstream into the brakes.

You could also consider investing in "Speedy Bleeders" which are bleed nipples with built in check valves. They use a gooey thread sealant instead of the Teflon tape I mentioned. Speedy Bleeders are available at most auto parts stores on the peg boards up front. All you need is the correct thread size.
 
Thanks for the advice on taking of the master cylinder, I was hoping it wouldn't come to that, I double checked the materiels the previous owner spplied me, all brake stuff is new or rebuilt, most rebuilt, I am wondering if I have a marginal master cylinder that doesn't compress fluid well be letting it slip by the seals, and maybe all the bleeding I have been doing made it worse, or I let a lot of air into the system when bleeding or when the bleed nipple was stripped.
I topped up on a regular basis, don't think I let the reservoir run low.

Doug, are the speedy bleeders the things that replace the bleed nipples?

Hope to get a chance to scratch my head and work on it more tonight after work.

Greg
 
Putting teflon tape on the bleeder threads shouldn't do anything at all. The bleeder is tightened before the pedal is let up, so there will never be any suction. With speed bleeders, you do let up on the pedal while the bleeder is loose, so you need the threads sealed.
 
Can you isolate wheel by wheel to see if it is at the wheels or in the master cylinder. IF the hoses are new they should be able to be crimped or plugged without much damage.

Try a pair of vice grips but with a wide applicator on the hoses, kind a like the welding vice grips, or put a couple of putty knives on each side of the hose and squeeze them together with the vice grips.

I'd try one wheel at a time to see if pedal pressure improves. If it doesn't then try blocking all four off.

Hopefully that will give you a firmer pedal. If not then you've ruled out the wheel cylinders/calipers.
 
Steve is of course correct about the Teflon tape.

I neglected to mention that I put the tape on my cars' bleed nipples because I have a bleed hose with a check valve in it. Sealing the threads with the tape allows me to use my check valve hose to bleed brakes "by myself". This means leaving the bleed nipple open during the "release" stroke. I should have mentioned this earlier and as I said, Steve's right... with the nipple closed no air will get past the threads.
 
Indeed, with a check valve you must seal the threads. Just like speed bleeders, there is the possibility of vacuum upon release of the pedal.
 
Thanks for the suggestions, those, some surfing and getting the Haynes manual and looking at the MC have given me some ideas about the problem fix, will not get back to it until tommorrow, will see how it goes, may try to get the speedi bleeds for the rear anyway.
 
My theory was that I had let the reservoir in the master cylinder (MC) go down too low, reintroducing air into the system, no such luck.

I get some pressure then it just goes away, pfft, like I am getting blow by in the MC seals. I actually get an air or maybe fluid noise when this happens.

So I took the MC off and checked it out, there appear to be signs of scoring or corrosion in the cylinder, but hard to tell because it is so long it is hard to see in there real well.

One more thing I noticed, the feed holes from the reservoir to the master cylinder the front one, towards the brake booster, has two holes in the master cylinder for fluid one is larger, one is tapered to a pin hole size (the other feeder is similar but has an extra hole for a retention pin.)

Anyhow, the pinhole was blocked with crud (the one closer to the booster), could this be my problem? maybe causing fluid not to fill the piston or something--I did notice that when I pumped to try to bleed the front brakes the piston fluid would all get sucked back on the upstroke, even when I closed the valve one the upstroke, suction was created which would still suck it back after I opened the bleeder valve again.
 
Well how 'bout that, RTFM--read the freakin (or substitute your word of choice depending on frustation level) manual.

In my own defense I didn't have a manual to start.

I always assumed bleed the right rear (farthest from master cylinder) first and work your way back to the closest front left brake.

Haynes Manual, Ch. 9 sec. 7(2)(this only applies to the later cars)

Bleed the caliper closest to the master cylinder first followed by the other front caliper, then the rear brakes

Thanks to this and easy bleed screws (I long ago ran out of any good will from the family as brake bleeders) with anti-seize applied to seal them a little better (instead of the suggested teflon tape but same principle) and a lot of bleeding I have good pedal, really good pedal.

Thanks for the tips and help.
 
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