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MG MGB Technical - bleeding the brakes?

I am going to attempt to bleed the brakes as I got air in the lines when I put new rotors and pads on. When looking in the manaul it said to disconnect the wiring on the pressure failure switch and then to unscrew the swith 3 1/2 turns. I located where they were talking about under the piece that is under the fluid reservoir but there was no screw (or switch) I could feel that there was a hole there and I'm afraid a previous owner may have broken it off. I'm hoping so much that I can skip this part of the proceedure and still bleed the brakes. Thank you so much for any advice anyone can give
Cathy P.S. It is a 1979 MGB
cathy hubbell

Hi Cathy The item/switch is to warn of circuit failure ( there are two, front and rear) it works by moving a small piston which then operates the switch . This piston does not function when both circuits have the same pressure, It is possible to get round this by using a pressure bleed system ( cheaply available from MOSS ) and open bleed valves at both front and rear at the same time, close them at the same time also, get a friend to help . Perhaps not the BOOK method but it works. To ensure the switch and piston are correctly placed involves removing the master cylinder. The piston often becomes stiff so if you disturb it you would probably have to overhaul the master cylinder. Jim
jim soutar

Huh? I've always been able to just bleed at the wheels, one at a time, rear left, rear right, front right, front left in that order. i've never had a problem.
don g

I'll echo Don's comment. But I'll add that bleeder screws such as "Solo-Bleed" and "Pegasus" sure make the job easier. I've been warned that they do not last, but I installed them last spring and did my bi-annual bleed by myself in about 20-30 minutes. So far, I think they're the greatest thing since Vice Grips. (The history of the world divides itself into two periods: before Vice Grips and after Vice Grips.)

Allen Bachelder

I use a mity-vac. Love it. I used it last week to reverse-bleed my clutch slave cylinder in about 15 minutes.

don g

don. on the 79b it has a dual circuit master cylinder. if when you bleed there is a differential in pressure between front and rearthe actuating piston for the system failure switch moves across to operate the switch, it does not reposition itself,. whilst this may not be importantwhist the switch is broken on cathy"s B if she ever fits a new switch problems could arise. bleeding the way I suggested avoids the problem tho it does not cure the switch warning failure. Jim
jim soutar

so it's different for my 69 gt and my 70 midget?
don g


I'd sure like to hear more about that Mity-vac. What model do you have, where did you get it, and about how much does one cost?
Tom Fisher

Tom- got it from victoria british, but i've seen them at harbor freight. dont know what i did without it! i can bleed the brakes by myself now, in a much shorter period of time. simple set up, you just pump it up and it creates a vacuum.

don g

Here is the link to Harbor Freight. They have the bleeder on sale $17.99

Ed Emery

Thanks, Don and Ed. I just ordered one from Harbor Freight. Including the shipping, it came to $25.98.
Tom Fisher

Don. unless US market MGs were fitted with dual circuit systems yes they are different. All UK MGs until 78 were single circuit and did not have the failure warning light fitted. It is quite possible to bleed in the manner you describe but the master cylinder will still have to be dismantled if Carly ever wants to reinstate the operation of the switch. Jim
jim soutar

Respectfully, I'm wondering how you got air in the lines from the work you did, Cathy.

If it was a screw-up, it's kind of fun to confess. We've all made funny goofs.

Still, if you have spongy brakes, just bleed them far to near (longest to shortest line) and go from there.

Never have bought a bleeder kit, but here's an alternative for you. Made from a salsa bottle and brake line. Have 2 for the different sizes of brake lines, but just as easy could have put 2 inlet tubes on the same top.
Height of simplicity (for me). Fill the bottle with enough water to cover the bottom of the LONG tube. Connect the LONG tube to the bleeder screw with rubber hose. Open the bleeder screw and start pumping the brake pedal. Long slow strokes going to the floor. The master cyl will pump the fluid into the water but isn't strong enough to suck backwards on the up stroke. 6 to 7 pumps and refill the master cyl. You won't be happy pumping it empty! When everything runs clear just tighten the bleeder screw and move on.

Another use is to test for vacuum. Break the vacuum line and put the bottle (with the water) in the line. The vacuum source side will be connected to the long tube and the vacuum operated item to the short one. If air is flowing there'll be bubbles in the water. Used to check brake boosters this was. Can do vacuum advances, etc.

Total cost: somewhere around $.25


T Lea

Remember that the reservoir has two chambers, not just one. So when you bleed the brakes you need to make sure both halves of the reservoir are filled with brake fluid.
Also, the switch that is missing from your car was simply removed from my car when I broke it trying to remove it. I just plugged the hole with a stainless steel bolt. Some on the list felt this wasn't safe, others disagreed. I haven't had a problem with it, and don't expect to. The car stops great, just as it should.
1977 MGB
randy olson

T Lea, I would have thought there would be hazards in using your methods, but you seem to have had good results.

Why not just start out with a bit of brake fluid in the bleeder bottle? If it does suck, at least you haven't contaminated your system.

I'm not sure I want to suck water into my vacuum lines, but may try your trick if the need arises.

Maybe I'm even more frugal than you, I just use a rag to squirt brake fluid onto, closing the bleeder before the bottom of the stroke. Soon as it stops sputtering, I try the brakes to see if things have improved. Next day, or before the next drive, repeat if needed.

Maybe Cathy put new hoses on, too. Good idea.

I hate to pinch my brake hoses, seems like a punishment they weren't designed to take.

Sorry for the randomness of my comments..

This system has worked for me for some 25 years. It's greatest when you have to work by yourself. My wife has an adversion to crawling beneath the car to open and close a bleeder screw (You don't think I'm going to do it I hope).

a) brake fluid costs money and water's free
b) You can easily see the brake fluid in the water when it starts to flow.
c) If you're sucking water out of the bottle and into your vacuum lines then you have the connections reversed on the bottle. The air is sucked through the water causing bubbles. Maybe I said it backwards. Old person I am, you know.

Tom Lea

I just purchased a set of speed bleeders for my '94 Nissan Sentra that I did a comoplete front brtake job to, pads,rotors,hoses. I was skepticle but the car is old and I wanted to flush the system. I put the bleeders on the fronts and cracked them open one at a time and had my wife pump the pedal because I wanted to see them working. easiest brake bleed I ever did, took like five minutes for each axel. I may buy a set for the MG.
MK Mike

This thread was discussed between 31/01/2008 and 05/02/2008

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