bleeding brakes

[Not_Anumber]

The new tandem master cylinder has arrived, I now just need some dry weather to fit it and re bleed the brakes.

Interestingly the replacement doesnt have a pressure differential switch or a place to fit one. This will make things much simpler.

[Charles Farran]

I don't recall ever seeing a dual line system brake master cylinder without a place underneath for the pressure failure switch!
All MGBs had the dual line system in the UK for the last two years of production with the U.S. market having it before.
Can you put up a picture?
Cheers,Charles

[Dave Wheatley]

The new tandem master cylinder has arrived, I now just need some dry weather to fit it and re bleed the brakes.

Interestingly the replacement doesnt have a pressure differential switch or a place to fit one. This will make things much simpler.

Is that the one from MGB Hive?
I'd be interested to know how it performs.

[Vic Butler]

Are you sure it's the correct one? As Charles said, all dual circuit brake master cylinders have the fitting for the pressure warning switch.
The part number in my parts list book is GMC 149 and it includes the reservoir which is held on the top of the cylinder by 2 screws. Have you compared it to the existing cylinder?

[Not_Anumber]

No you are absolutely right Vic. There was a label in the way on the packaging, Ive spotted the extra bung where the switch goes now.

Reading the two sections in the Haynes manual again Im still not clear if I should first bleed the brakes with the switch fitted (furthest wheel first) and then stop to unscrew the switch by 3 1/2 turns and bleed again, this time starting with the closest wheel first.

Or is the first operation not necessary ?

[Charles Farran]

Hi Chris,
You have to unscrew the pressure switch first by 3 1/2 turns - see attached extract from the Repair Operation Manual (AKM 4070).
Cheers,
Charles

[Vic Butler]

Front offside, front nearside , then the rears. No order bleeding the rears in the manual.
The brake pedal must be pushed rapidly right down .allowing it to return unassisted. This procedure is repeated until no air bubbles are present. The same procedure with the other brakes. Then, if the electrics are connected the handbrake is applied and the light comes on. It goes out with the handbrake released then press the brake pedal down hard. If the light illuminates you have to do the procedure all over again and the switch must be screwed out the 3 1/2 turns before bleeding.
If pressure or vacuum bleeding there's no need to undo the switch because the differential piston won't be moved as the brake pedal won't be applied.
Best to get the proper Repair Operation Manual.

[Charles Farran]

Vic,
I suspect if filling fluid to the system & bleeding for the first time, then it maybe essential to unscrew the switch, otherwise if the differential piston isn't in the middle position , then the plip of the button of the switch may possibly be wedged in one of the two indents & thus prevent the piston moving into the correct middle position where there will not be any indents......
Cheers
Charles

[Vic Butler]

Agree Charles. That thought hadn't struck me. The brakes would then have to be bled by the traditional method otherwise the piston won't centralize. Thanks.

[Not_Anumber]

So am i right in thinking the consensus is:

bolt the new master cylinder to the servo in place of the old.
Leave the switch either removed entirely or 3 1/2 turns undone.
fill the reservoir with fluid
connect the unions (carefully one at a time transferring from the old cylinder to limit fluid loss)
bleed the brakes; Front offside, front nearside , then rears
screw in the pressure differential switch.
continuity test across the switch. If pushing down hard on the pedal changes the continuity then unscrew the switch and re bleed.