bleeding brakes

Technical MGB discussion
Vic Butler
Posts: 565
Joined: Wed Dec 21, 2016 6:07 pm
Forename: Vic
Surname: Butler
Location: North West Hampshire

Re: bleeding brakes

Post by Vic Butler » Wed Sep 30, 2020 10:04 am

Actually Ian, I checked the manual since that post and it's the pressure warning switch that's unscrewed 3 1/2 turns before bleeding the brakes. By the time I had realised the error you'd posted your reply so apologies.
The shuttle valve itself will move as each circuit is bled then should centralize itself. This is checked by switching the ignition on then applying the handbrake which will illuminate the warning light. Releasing the handbrake will extinguish the light then pushing the brake pedal down, the light should stay off. If it lights up then the valve isn't central and the brakes must be bled again. I was lucky with pressure bleeding and the light didn't illuminate when I pressed the brake pedal. What did happen was the pressure warning switch broke when I tried to loosen it. With the aid of a mirror I was able to tap the remains out of the underside of the master cylinder and fit a replacement. No fluid was lost during the operation.
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine

Ian Fozzard
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Joined: Fri Mar 27, 2015 8:18 am
Forename: Ian
Surname: Fozzard

Re: bleeding brakes

Post by Ian Fozzard » Wed Sep 30, 2020 10:26 am

Thanks Vic. Clearly the brakes on the later Bs is quite a bit more complex than the early cars. Your information may be of some help to the OP who seems to have hit an obstacle with his car.
Always difficult to diagnose and offer solutions without seeing the vehicle, but at the present time I think a pressure/vacuum bleeder might offer the best approach.

Ian F
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear

Not_Anumber
Posts: 60
Joined: Tue Sep 22, 2020 9:33 am
Forename: Chris
Surname: Silk

Re: bleeding brakes

Post by Not_Anumber » Wed Sep 30, 2020 5:47 pm

Got there now.
I left it to settle for a couple of days then opened one of the rear wheel bleed nipples and pumped the brake pedal whilst leaving the nipple open. Lo and behold it was now expelling some brake fluid at last. I repeated the same on the other rear wheel and the fronts then bled the air bubbles out normally with the help of my long suffering daughter on pedal duty. I then noticed the nut was missing from one of the front flexible hoses as it it had only been fitted temporarily before. As soon as I can find a nut to fit that I'l re bleed that corner, wire brush the disks and that should be the brakes back to working order.

Now to work out what goes to where on the wiring loom, how to refit the radiator, expansion tank and oil cooler. :)

Ian Fozzard
Posts: 517
Joined: Fri Mar 27, 2015 8:18 am
Forename: Ian
Surname: Fozzard

Re: bleeding brakes

Post by Ian Fozzard » Wed Sep 30, 2020 5:54 pm

That's good! Sounds like it just needed a little more brute force!

Ian F
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear

Not_Anumber
Posts: 60
Joined: Tue Sep 22, 2020 9:33 am
Forename: Chris
Surname: Silk

Re: bleeding brakes

Post by Not_Anumber » Thu Oct 01, 2020 4:08 pm

Looking at that reference to the pressure warning switch.

Is the objective of unscrewing this just to ensure that the warning light stays out or does air get trapped above the switch which would then be likely to move into one of the brake lines ?

Vic Butler
Posts: 565
Joined: Wed Dec 21, 2016 6:07 pm
Forename: Vic
Surname: Butler
Location: North West Hampshire

Re: bleeding brakes

Post by Vic Butler » Thu Oct 01, 2020 5:00 pm

It's to disconnect it from the shuttle valve. No air gets in and no fluid escapes even when the switch is removed as I had to do. 3 1/2 turns per the official workshop manual.
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine

Not_Anumber
Posts: 60
Joined: Tue Sep 22, 2020 9:33 am
Forename: Chris
Surname: Silk

Re: bleeding brakes

Post by Not_Anumber » Thu Oct 01, 2020 5:31 pm

Im not clear of the purpose of disconnecting it from the shuttle valve. Is this just to ensure the brake pressure warning light behaves correctly or will it have dislodged a pocket of air into the brake lines ?

Unfortunately i dont understand the significance of the shuttle valve so not clear on the the worst case outcome of not doing it right.

Vic Butler
Posts: 565
Joined: Wed Dec 21, 2016 6:07 pm
Forename: Vic
Surname: Butler
Location: North West Hampshire

Re: bleeding brakes

Post by Vic Butler » Thu Oct 01, 2020 5:46 pm

It's to disconnect the switch from the differential piston, not shuttle valve as I thought. There's a special way to bleed the brakes as I thought so a proper workshop manual is essential. The differential piston is operated by the master cylinder piston.
Now, if the system is pressure or vacuum bled, the master cylinder main piston isn't moved so there's no need to slacken the switch.
I checked the way of bleeding Series 3 Land Rover dual circuit brakes. The warning switch incorporates a shuttle valve and full pedal travel mustn't be used for brake bleeding to avoid decentralizing the valve.
The best thing to do is study the workshop manual and when the electrics are connected check the light doesn't stay on. If it does, the brakes must be bled in accordance with the manual to recentralize the differential piston.
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine

Not_Anumber
Posts: 60
Joined: Tue Sep 22, 2020 9:33 am
Forename: Chris
Surname: Silk

Re: bleeding brakes

Post by Not_Anumber » Thu Oct 01, 2020 9:20 pm

Will the brakes be spongy or not be fully effective without this ?

Vic Butler
Posts: 565
Joined: Wed Dec 21, 2016 6:07 pm
Forename: Vic
Surname: Butler
Location: North West Hampshire

Re: bleeding brakes

Post by Vic Butler » Thu Oct 01, 2020 10:13 pm

It's more than likely only one circuit will be operative so 2 wheels will have no brakes. It must be done to the letter,lives could depend on it.
If you're in any doubt, get the brakes fixed by someone who knows about it. Brake work must not be skimped.
You can't bleed the brakes properly until the electrics are working because there's no way of knowing whether the differential piston has been decentralized without the warning light working.
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine

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