MG Sports Cars

engine swaps and other performance upgrades, plus "factory" and Costello V8s

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Actionjack
Jack Abranovich

(4 posts)

Registered:
03/02/2023 06:12PM

Main British Car:


Master cylinder issue
Posted by: Actionjack
Date: September 28, 2023 04:28PM

Just replaced everything on my 73 MGB,Master cylinder,all lines,hoses,calipers rotors,wheel cylinders,shoes,have real good preasure at front calipers,front brakes work great,can't get much preasure going to real drums,not mutch at master cyl. or at rear bleaders,no air,pedal soft for about 2" then nice and hard,bench bled master,gravity bled,odd that the soft feel at first in pedal is front brakes,that work great,then the good firm pedal is when the rear brakes are being applied and not much preasure,can turn rear wheels with brakes on,what about check valves shown in master cyl.can one be sticking,don't know what to do next unless new master cyl.is junk,been a mechanic for 40+ years,never had this issue.


DiDueColpi
Fred Key
West coast - Canada
(1365 posts)

Registered:
05/14/2010 03:06AM

Main British Car:
I really thought that I'd be an action figure by now!

authors avatar
Re: Master cylinder issue
Posted by: DiDueColpi
Date: September 29, 2023 02:14PM

Hey Jack, pull the rear brake line off the master. Screw a plug into the port on the master. Your parts store should have blank plus in stock. And then try the brakes. If you have a good pedal then the master is good.
Next move onto the 4way brake pressure switch. I'm going to guess that this is your problem. The switch, or valve really, is designed to shut off flow to either the front or the rear brakes when it senses a failure. This will trip the warning switch and light up the warning on the dash. You need to re-center the valve. You can do this by opening a front breeder and giving the pedal a stab to turn the light off. Then bleed the rears gently so as not to retrip the valve.
If that's not it, then grab your trusty plug, the one that you didn't throw away after the master tested fine, and plug the rear T one side at a time to isolate each rear brake.
All of that should point you towards the problem. If not hurry up! the lovely Lynne just brought me a Scotchy Scotch so my usefulness is time sensitive. :-)

Live like you mean it.
Fred


Actionjack
Jack Abranovich

(4 posts)

Registered:
03/02/2023 06:12PM

Main British Car:


Re: Master cylinder issue
Posted by: Actionjack
Date: September 29, 2023 04:38PM

Fred,I have a good hard pedal,I did away with the 4 way.valve as I put an 8" Ford drum brake rear in it,I will try the plug trick,a buddy is dropping of his power bleeder tomorrow to try also,thanks for the response.


BlownMGB-V8
Jim Blackwood
9406 Gunpowder Rd., Florence, KY 41042
(6469 posts)

Registered:
10/23/2007 12:59PM

Main British Car:
1971 MGB Blown,Injected,Intercooled Buick 340/AA80E/JagIRS

authors avatar
Re: Master cylinder issue
Posted by: BlownMGB-V8
Date: September 29, 2023 05:04PM

If you didn't bench bleed the MC before install that is likely your problem.

Jim


DiDueColpi
Fred Key
West coast - Canada
(1365 posts)

Registered:
05/14/2010 03:06AM

Main British Car:
I really thought that I'd be an action figure by now!

authors avatar
Re: Master cylinder issue
Posted by: DiDueColpi
Date: September 29, 2023 11:55PM

so.... you're good and hard, you have 2 inches of travel and no pressure in the rear. but there's no resistance.
We're talking about a car...right? :-)

yeah I know I'm in trouble.


DiDueColpi
Fred Key
West coast - Canada
(1365 posts)

Registered:
05/14/2010 03:06AM

Main British Car:
I really thought that I'd be an action figure by now!

authors avatar
Re: Master cylinder issue
Posted by: DiDueColpi
Date: September 30, 2023 04:48AM

In all seriousness. if the master is solid, it has to be something in the drums.
I would strongly encourage you to install a pressure regulating valve. Without it you simply have a single line system with all of it's failure issues.


Actionjack
Jack Abranovich

(4 posts)

Registered:
03/02/2023 06:12PM

Main British Car:


Re: Master cylinder issue
Posted by: Actionjack
Date: October 05, 2023 08:26PM

Update,I got my buddies power bleeder,put preasure in the system,no air in the rr w/cyl.,went to left side,one little burp of air and I then had rear brakes,must have been somewhere before T on rear end,have good pedal and brakes all around after a manual bleed afterwards,thanks for the help.



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