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MG Midget and Sprite Technical - Midget Mk2 Dual Master Pedal Feel

Next dumb question that I think I know the answer to already...

Helping out with a mk 2 Midget with the combined brake and master cylinder and disc brakes.

I drove the car a couple of weeks ago and the brakes were none existant, so I suggested to the owner they should be the first thing to fix.

They have told me they have fitted various new bits and thoroughly bled the system and correctly adjusted the drums.

They say the brakes are now working fine, but to me the pedal (whilst not moving) still feels really soft with a low, soft "biting point".

Is this a quirk of this setup? or should they feel much the same as later, split cylinder setups? I have only ever driven later types, where the pedal has a fairly short, firm stroke.

Cheers,
Malc.

Malcolm

Malcolm
It depends upon what, was not, changed by the person doing the conversion unless your Mk2 is the 1098 engined version that came with discs. So much gets changed over time. The new bits may, may not, be correct for the application. I have seen some awful home mechanic rebuilds in the past few months as well as a couple of awful professional jobs.
Alan
Alan Anstead

Thanks Alan.

In theory it is an original disc brake car (1964) But it's such a mish mash, who knows.

Wrong parts would be too small a bore master or too large a bore on the rear cylinders, right?

And I guess there is no easy way of telling the difference?

Malc.
Malcolm

A good combined MC disk brake setup should feel rock solid with a high pedal.
A common mistake is fitting an MGA MC which is a 7/8 bore instead of the correct 3/4 bore.
Then fitting a drumbrake MC with a residual pressure valve can mess things up as well.
So plenty of options for getting it wrong ;)
O K

I fitted disc brakes to the front of my frogeye and retained the drum brake MC and wasn't aware of the residual pressure valve, it worked fine and I was never aware of any issues with the brakes as a result. I was aware of the MC size difference but it just resulted in a heavier brake pedal but it wasn't an issue. The rear cylinders had been changed to the 3/4" MM cylinders because that's what the local factor had when I did the rebuild. The MC was later changed to the disc one when the drum one needed replacing and the drum one was NLA.
David Billington

OK, so it seems the conclusion is that something is not right (as I thought). I will gently advise as such ;-)

Can't see why a 7/8 cylinder instead of 3/4 would cause such issues. 7/8 would make the pedal stroke shorter and stiffer?

But now it is not starting (fuel pump stopped working amongst seemingly other issues) so they don't need the brakes anyway!
Malcolm

The other thought is wear on the brake pedal/master pushrod allowing more slack thus increasing travel.
Bob Beaumont

I was just trying to indicate that there are plenty of possible issues when all seams perfectly well.

Off course a soft deep pedal should be sought in air in the lines, slack in the pedal linkage (more intricate in the combined MC cars)
Though the wrong size rear cylinders are a logical suspect as well (you don’t always get what you order)
O K

Malc

Some useful background info on differences between drum front braked cars with 7/8 inch dual cylinder and front disc braked cars:
http://www.gerardsgarage.com/Garage/Tech/DbrakeUg.htm
&
http://www.gerardsgarage.com/Garage/Tech/DBrake/TandemMC_Part1e.htm
&
http://www.mg-cars.info/mg-midget-sprite-technical-bbs/refurbishing-twin-master-cylinder-2017062111263131648.htm

Apart from different master cylinder and rear wheel cylinder bore (3/4 inch) the disc braked cars with dual master cylinders have different master cylinder pushrod lengths and different valves in the brake master cylinder.

For a long time only 7/8 inch bore dual master cylinders were available new (fit MGAs and some old forklifts apparently) so for 3/4 inch was rebuild only, or perhaps people fitted what they could get i.e. 7/8 inch. You could quite recently get a new AP Caparo (now '921 Engineering') - i.e. genuine Lockheed - 3/4 inch dual master cylinder from Powertrack Brakes Ltd (https://www.powertrackbrakes.co.uk) and possibly David Manners. NB there have been some faults reported with TRW made 3/4 inch dual master cylinders which are widely available, hence some folk (like me) going for and tracking down the AP Caparo one.

Cheers
Mike
M Wood

This thread was discussed between 30/08/2020 and 02/09/2020

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