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MG MGB Technical - brakes !!

79mgb. last friday and saturday i rebuilt the brake master cylinder (booster not touched). i am positive the seals are installed correctly, etc !
saturday and sunday had great brakes. didn't drive on monday.
today, tuesday no brakes, pedal to the floor.
what did i do wrong? is there a trick to bleeding?
what else could/should be done ?
all help gratefully accepted and appreciated.
thanks.

john
john sutter

Did you bench bleed the Master before you put it back in? How did you do the bleeding? Generally you start at the wheel that is further (hose and tibing length, from the MC, then work towards it. Also if your car has the brake pressure failure switch you need to loosen the switch in the body (H shaped piece the four pipes connect to)to bleed it too.
william fox

william

no !, damn it, i did not bench bleed it, nor did i bleed at the brake failure switch. i hope i can do that successfully with the unit still on the car.
i'll do that tomorrow.
watch this space.
thanks

john
john sutter

I had a problem with bleeding the MC on a 73 BGT I bought that had sat for awhile. Instead of removing it I just disconnected the lines and bench bled with it in the car- had my son pump the pedal. Get an old brake line with a fitting, cut off the line about an inch or so from the fitting and attach a piece of clear plastic tubing. Screw the fitting in one ofthe outlets and direct the tubing into the reservior. I ended up bleeding my whole system using about ten feet of tubing.
william fox

Never bothered bench-bleeding and never noticed the lack of it. When the pedal went to the floor was there fluid in the m/c? If not where had it run out from? If there was, and repeated pumping produced no brakes, it sounds like the piston seals have failed.
Paul Hunt

william
brakes getting gooder, thanks for your time.

i found your use of a fitting and 10 feet of tubing intriguing, but the rear wheel cylinders are mounted
upside down so that the bleeder fitting is at the bottom of the cylinder. thus if you bleed at the fitting you can't remove the air bubble. i usually bleed at the line and let it drip.
this is my second 79mgb and both had the rear cylinders mounted incorrectly, in my opinion. and, you can't invert them. ah, the british car maker.

comment, or have i missunderstood you ?

john
john sutter

No,sounds like you have the idea. I think its common though for the bleeder to be on the bottom. Try getting an EZ Bleed kit and bleed the brakes from the bleeder valve forward. Did you see Paul Hunt's post about the seal failure?
william fox

John. Which Congressional District do you represent? Les
Les Bengtson

As far as rear cylinders go when the brakes are released the backs of the two pistons are pulled into contact by the shoe pull-off springs. Because each piston has a chamfer this leaves just a very narrow channel between the inlet and the bleed, which really is no different to a length of pipe.
Paul Hunt

william, paul, and les
thanks for the info on the construction of the rear wheel cylinders. on my previous cars, big healeys, the bleeder is above the line where it should be, so i assumed, incorrectly------.
in my original post i said i had rebuilt the m/c, i.e.
new seals etc. so, they should not have failed, nor do i believe that the seal lip has doubled over. i had excellent brakes for two days, so that it would have had to happen spontaneously after a period of good brakes.
right now i have brakes but i think more bleeding is necessary to reduce free pedal. i will go with that assumption, unless you have a better idea.

les, "congressional district" ? please clarify, don't torture an old man trying to figure it out. i would be very happy to answer whatever "in joke" this is. if it is that, i missed it.

john
john sutter

As far as bleeding goes there seem to be several pet ways to get all the air out of the system as 'normal' bleeding seems rarely to work totally. In my case I use an Eezi-Bleed in its normal configuration and bleed the longest run first and the shortest last. Then my beautiful assistant stands on the pedal while I rapidly open and close the front (the backs don't need it) bleed screws. This always gets an extra 'lump' of air out which the slow and steady bleeding from the Eezi-Bleed seems to leave in the pipes and cures the 'long pedal that can be pumped up' symptom.
Paul Hunt

my brakes were working just fine 2 days ago and then i was driving my 68 mgb roadster and the brakes went out. the breaks are brand new and i just replaced the master cylender. does anyone know how i can fix this

thanks
scot
Scot

I have found that leaking rear cylinders can
give the same symptoms as a bad MC. In fact,
a relatively small leak in the rear cylinder
can let a lot of air into the lines giving
symptoms as described. Double check your rear
cylinders and make sure they're not moist with
brake fluid.

Actually, double check your brake lines and
unions for small leaks also.

As to bleeding, I've never bench bled either,
and have always had good luck with the eezibleed.
Without a pressure bleeder, I can see why
bench bleeding is necessary. Good luck!
Ronald

Scott - did you read the previous postings in this thread?
Paul Hunt

This thread was discussed between 04/12/2001 and 09/12/2001

MG MGB Technical index

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