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MG Midget and Sprite Technical - Brakes

I have just got the car moving after a long rebuild and the rear brakes seem to be locking up before the front so is there a brake balance valve or bias valve that I should check the master cylinder and wheel cylinders were all replaced
Any help would be great
R Deans

Hi

There is no bias or brake balance valve. Did you replace the flexible hose at the rear? This can cause brake problems at the rear when it collapses internally.
Bob Beaumont

If its after a rebuild with new parts, check for run-out on the front discs.

I seem to recall my 1500 did have a bias valve on the rear brake line, but it was a very late production one. It may not even have been an original fitment.This probably doesn't apply to your car?
GuyW

A fundamental question before I throw in a guess... how have you tested and come to this conclusion?

Loads of things it could be, but my first thought was have you fitted the correct rear cylinders?

GWC1102, pre-1500, 3/4" (19mm)
GWC1129, 1500 onward, 11/16 (17.5mm)

Smaller rear cylinders would bias the braking rearward.

Cheers,
Malc.
Malcolm

other way round Malc--smaller cylinders=lesspressure
William Revit

Damn, I was so close to sounding smart this time! ha ha ha!

Time to go back to skool!

Malc.
Malcolm

Is that right? Smaller wheel cylinders receiving the same quantity of fluid on a stroke of the pedal so piston will travel further. Only the travel is limited by the shoes so pressure must increase as pressure is measured per unit area. Or is that all crap!

Thought I was coming to Malc's resuce, but now my brain hurts!
GuyW

I used to remember that it was always opposite to what I thought it should be. But then I think I confused myself when typing what I did, I had it right in my head, then thought "yeah, but it's always the opposite" so ended up the opposite of the opposite! ha ha.

Consider the brake fluid pressure, rather than the volume of fluid flow Guy. The brake force is given by the fluid pressure times the area of the piston.

The front brake pistons are always the same. So for a given brake line pressure they will generate however much braking force.

So if you reduce the size of the rear cylinders, but keep the brake fluid pressure the same, your front brake force is the same, but the rears is less, as force = pressure x area. Thus less rear bias.

But with that, the pedal stroke is shorter as you need to move less fluid to achieve that given fluid pressure.

Does this make sense?

Cheers,
Malc.
Malcolm

Yep, got it! I convert it into bicycle chain sprockets. The powered end (crank) is equivakent to the brake master cylinder, whilst the slave end is the driven, so the wheel sprocket on the bike or the brake actuating slave cylinder on the car. Keep the same chainwheel sprocket but reduce the rear sprocket and the wheel turns faster but with less force to help on the hills = less pressure acting against gravity.
So you travel further, but with less power

Hmm, well I know what I mean!
GuyW

I used to use 1500 wheel cylinders on the race car to reduce rear braking, although I ended up going down another size to Cooper S.
Dave O'Neill 2

Are the locating pins on the cylinders not in different positions for the alternative sizes ? Meaning you have to redrill backplates/ cylinders to fit the alternative size ?

R.
richard b

Richard

You are correct.
Dave O'Neill 2

Thank you all for the advice
The big problem I have is the car is a mix.
It is a 1500 body with I believe earlier front and rear running gear so I think I will start by renewing all the brake hoses and see what happens
R Deans

This thread was discussed between 28/06/2023 and 08/07/2023

MG Midget and Sprite Technical index

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