Welcome to our resource for MG Car Information.



MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB GT V8 Factory Originals Technical - Brake lines and heat

Anyone have brake problems related to heat from the headers after doing a V8 conversion? I installed a new stock MGB master cylinder in my 1977 MGB-V8. PROBLEM: the brakes lock up. This occures while the car is idling from a cold start. After about 10-15 min the brake pedal becomes firmer and the brakes, especially the fronts, become very tight on the rotors (these are new calipers that worked fine before the new M/C) . I have disconnected the vacuum from the boost and that didn't help(ok a little). The brakes can be relieved by cracking the brake line at the master cylinder. The calipers immediatly release the rotor.(these are new calipers that word fine before the new M/C).
The lines are in the original positions and run about 1 and 1/2 inched from the header. It must be heat. Any suggestions? If it is heat why wouldn't the expanding fluid simply flow back through the M/C into the bowl. I have actually put on a second new M/C thinking there must be something wrong with it.

I had this same thing happen to me on a 78 I had. I replaced the MC with a new unit and the font brakes would lock down after a few stops. It ended up being a defective MC. This is probably your problem. On my MGBV8 the lines are about 1-3" away from the RV8 headers and I have never had a problem. I am using SS brake lines. Good luck
Evan Amaya

I have had 4 V8's, 2 with block hugger headers, 2 with the RV8 style, & have never had a problem as you describe. I have taken the precaution to bend the hard lines a bit further away from the headers, but just what I could do by hand, without extending or disconnecting the lines. I have always replaced the rubber flex lines with Airquip type lines on all my MG's.
Jim Stuart

Thanks guys.
The problem is solved. Turns out that the pushrod from the vacuum boost to the M/C was a bit to long (half the thickness of a washer). When the brakes were locked I loosened the M/C nuts a little and the calipers released. I guess I'll have to grind the pushrod or make a spacer. For now the washers I'm using as spacers are working. A local brake guy suggested this as the most likely cause for the problem I described. He also said the proximity of my brake lines to the header,1", shouldn't be a concern.
It looks like I'll be back on the road again.


You should find that the push rod is adjustable - the end of the rod is threaded to the rest of the rod. With the master cylinder off you should be able to expose enough of the rod to adjust it simply by getting someone to push the brake pedal down for you.

Geoff Richmond

According to the manual, the brake servo rod stem length needs to be set at 0.400" protrusion from the servo shell otherwise, as you have experienced, it could prevent full retraction of the piston in the M/C.
The pushrod is adjustable, as Geoff has mentioned.
In my conversion with blockhuggers, underhood heat doesn't appear to be a factor in brake performance other than contributing to brake fluid expansion that vents out the M/C cap if the reservoir is overfilled.
Graham Creswick

Thanks again for the tips.
I didn't know the servo push rod is adjustable. I'll have to a adjust it to 0.4" as Graham indicated. Then I won't have to stop every 10 miles the bleed off the brakes and I can gety rid of the spacer I put in.:)


This thread was discussed between 11/06/2004 and 14/06/2004

MG MGB GT V8 Factory Originals Technical index

This thread is from the archive. The Live MG MGB GT V8 Factory Originals Technical BBS is active now.