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MG Midget and Sprite Technical - TRW master cylinder faults???

Can anyone tell me exactly what is, or are the perceived faults on some TRW tandem master cylinders?
I have just managed to strip the thread on one of the ports of my Mk3 Sprite’s m/cyl, but I have a TRW one which came with an abandoned project that I bought, so I thought I take a chance and fit it, but am a bit wary if there are any safety critical issues.
Thanks. Bernie.
b higginson

Bernie

It might be worth a look at the MGA section archive, as they use a similar cylinder. I thin kit has been discussed a few times.

The 'live' BBS is anything but, ATM.
Dave O'Neill 2

Do you mean the dual Clutch/Brake master? There is nothing 'at length' (tandem) about them.

If you search the archives here, the TRW dual masters seem to have gone through a bad patch about 10 years ago. I seem to remember it was to do with the valve in the brake bore.

With that in mind I fitted an AP Colparo one a few years ago as a replacement for an old TRW one. Then followed lots of clutch problems - which I knew would not be caused by the 'good' new master cylinder. Long story short, the fault was a poor seal in the master's clutch bore. A replacement seal was the fix but it took many goes to work to that point.

My view is there is little difference in them, especially as the TRW problem seems to have gone away.
Karl Thompson

Thanks for that,Dave. there was quite a bit of info about residual pressure and even using two gaskets on the end plate, but cutting them so that the piston butts up against the plate not the gasket, allowing the piston to clear the return hole.
I had a good look inside the cylinder and pulled out the valve in the bottom of the brake side. It looks as though it may not work as well as the old one, so I used all the internals from my old cylinder including the original valve and with new seals in the TRW body.
I will be filling and bleeding the system tomorrow, so I’ll report back when I’ve done it.
Karl. I used the term tandem as a general description because both brake and clutch bores are in the same body.
Bernie.
b higginson

I fitted an AP Caparo about 4 years ago and it’s been fine . I got one as there were lots of rumblings about TRW. I think it’s down to luck as it’s difficult to know when they were manufactured anyway when you buy one .
Mike Fairclough

Bernie
I fitted a TRW tandem m/c to the Frogeye (3/4" version so suitable for front discs) and it's been fine up to now.
Bill B

Bernie
The problems with TRW dual master cylinders were leaking and or residual pressure. I had one poor chap contact me after having had three fail on him.
I suggested that he had his original master cylinder sleeved by Past Parts.
More detail can be obtained from the excellent MGA Guru website.
I did raise the issue on a TRW stand at a Mechanix Show, Sandown,some years ago but there was little interest.
Alan Anstead

Bernie, I bought one a good while back. The first one leaked straight out of the box and the replacement originally caused the front brakes to bind which meant adjusting the pushrod length until it freed. As mentioned above a lot on the MGA forum about them which I wasnt aware of until after I had purchased mine. I wouldnt buy another one.

Trev
T Mason

About ten years ago I bought a TRW m/c for my LandRover Defender.
It was rubbish. It had swarf inside it and leaked from three separate places.
It took a while but eventually I got my money refunded. In the meantime I bought one from Rimmer Brothers. I don't remember the brand but it worked fine.
At the time the bloke at Rimmers told me that TRW bought in their parts from China. There were several "brands" that were all made in the same factory and were identical barring the packaging.
He also mentioned that TRW had recently been bought by a German outfit called ZF. I remember that because I thought it was odd - ZF in my experience were an American gearbox maker.
But hey-ho...
They might have upped their game on quality control since then but I'd prefer not to chance it if there was a known decent brand available.
Just my 2 cents.
Greybeard

Grey
Here about 5-6 years ago I went to replace the oil pump on a friend's MGA (1800)engine--- The pump gear on his had come loose on the shaft, it was one of those that had the gear pinned to the shaft-
So here's this brand new TRW pump sealed in a plastic bag with oil floating around in the bag, which sort of gives out a vibe that it's been lubed and tested---BUT
-So I poked the drive shaft/gear into it and squirted some oil in it and went to give it a prime up before fitting it and it locked up---pulled the end plate/pickup housing off for a look and found a fairly large (rice size) piece of aluminium in the gears. Cleaned it all up and oiled it and it spun freely-all good but when looking at the end piece before reassembling found where the piece came from----Normally the pickup tube is drilled through but not this one the pickup bore was actually cast into the housing and up at the pump end the hole was 90% blocked with flashing from being cast, the remaining tiny hole was where the piece that was in the gears of the pump came from- so into it with a burr and opened the hole up to match the pump and all was well---but it could have been a disaster trying to get oil pressure up--If the pump hadn't jammed up i probably would have poked it straight in as was thinking it'd been run/tested before it got packaged---It was a reminder to me to not trust anyone--do it yourself or don't do it at all
willy
William Revit

Update. The TRW master cylinder has been on for a month without and problems with either clutch or brakes
Thanks for all the comments and advice.
Bernie.
b higginson

I fitted a TRW 3/4” dual master cylinder today, seems ok so far, although not been on a run yet.
The reason I changed the master cylinder is that on a run last Sunday I lost clutch operation, pedal seemed to loose pressure. If I pumped the clutch pedal very slowly then could get a few gear changes. The original cylinder was a Lockheed although I did change the seals quite a few years ago ( maybe 8 or 9 years, not too sure!) as the originals had failed while on a trip to Germany.
On returning home on Sunday I bled the clutch and the fluid (DOT4 synthetic) was black. On stripping the cylinder there was lots of gunge inside and the 2 very small holes in the cylinders were blocked. Had the seals perished over time? However the brakes worked fine though and no visible leakages.
I will thoroughly clean the cylinder body and fit new seals then keep this as a spare.
Les Robinson

Went for a short run earlier today, all seemed ok.
There was talk of residual pressure, what are the signs of this?
Looked at my old cylinder bores and seals but couldn’t see anything obviously wrong. Need to check my old seals against new ones.

Anyway, next problem is squeaking 1/4 elliptic rear springs - driving me nuts!
Les Robinson

Les, the residual pressure usually shows up as the front brakes binding. If its as bad as mine was you will soon know about it.

Trev
T Mason

I have noticed that if I have applied the brakes then release the pedal there is very slight drag on the disc pads, I can still spin the wheels by hand but definitely a slight drag. After a while this goes away and the wheels spin much easier. Is this the master cylinder residual pressure or the calliper pistons not moving back - guess there is no load to push them back? Car seems to roll easily enough. Maybe it’s always been like that, can’t remember.
Yes it’s a dreaded TRW master cylinder!!
Les Robinson

Forgot to mention, the passenger side has slightly more drag than the drivers side.
Les Robinson

Les,

IIRC it's the square seal in the brake caliper that pulls the piston back, in use it lozenges when the brakes are applied and when the pressure is released they go back to their square state pulling the piston with them, the resulting gap is tiny but enough to not drag. I have had slight dragging in the past and that seemed to be down to worn discs and pads, the pads weren't parallel any more and the disc was slightly below minimum spec so I assume it allowed the piston to tip over slightly and not pull off entirely, changing the discs and pads cured the problem.
David Billington

David
Thanks for that.
My discs and pads were renewed 2 1/2 years ago and look absolutely fine.
With all the past issues with the TRW master cylinders my initial reaction is to assume that is the issue. It’s just that I can’t say for sure if there was not the same issue previously.
Les Robinson

Les, mine was more than a slight drag. You can soon see if its the MC by adjusting the pushrod a bit and seeing if that cures the drag.

Trev
T Mason

Trev, I have set the pushrod clearance to the recommended gap - 1/32”.
After a while the drag goes away.
Les Robinson

Trev
How did you finally resolve the problem?
Les Robinson

You could try cracking open the bleed nipple and see if the drag goes away.
Dave O'Neill 2

Les, increasing the gap cured it in the end.

Trev
T Mason

I was wondering (hoping!) that as it’s a new cylinder with new seals then maybe the seals could be a bit stiff until they bed in a bit. The clutch side seems to be ok but this gets a lot more piston travel, so moving the seals more.
Last option is to buy a new O.E. cylinder - (AP brand).
These were out of stock when I ordered the TRW. Or have my old one sleeved - don’t know anymore local to me though (Preston area).
Away in the car this weekend (Prescott hill climb) then 10 days after that going to France, ideally want it sorted for France trip. Did just over 100 miles on Monday (Gorsworth show) and it seemed ok other than me knowing the brakes were binding a bit, they didn’t overheat.
Les Robinson

The other thing I found with mine was that when I had the issue putting your foot under the pedal and lifting it freed them off. That will tell you if it is residual pressure or not.

Trev
T Mason

I have the recommended clearance between the push rod and stop, 1/32” (0.83mm).
This is the gap measured with feeler gauges when the brakes are free. After depressing the brake pedal hard a few times then the gap is the same. This would suggest to me that the piston is not sticking otherwise the gap would be larger.
The brake pedal is really hard, hardly moves so guessing that the piston moves very little anyway - is this right?
I seem to recall that previously the pedal was more squashy.
Les
Les Robinson

Hard pedal is good! Squashy is concerning to it is good to for that to have changed.

That there is more drag on one side than the other points towards the cause being at the wheel end - if is was just the m/cylinder then both would be the same.
Most common cause is sticky caliper pistons, but also a brake hose failing internally can act like a valve.
Sticky pistons in either calipers or wheel cylinders is quite common on cars that don't do a lot of mileage.
Check there is no rust on the pistons, a common cause of sticking.
If OK, try working the pistons in and out a dozen or more times (having first got any dirt off them). That usually frees it up.

Having said all that, I suspect you may not have a drag issue at all if you are just checking the wheels for drag with the car in the air. You note that the drag is slight and that after a while the drag goes away. As David noted above retraction of the pistons is via the caliper seals acting like a spring when compressed. A bit of effort is needed to push the pistons back (try it manually, you'll see what I mean) and as the seals decompress the amount of push decreases. Just like any spring. So the last bit of push will be slower than the first.

I haven't ever measured it, but I suspect that 90+ % of the piston retraction would happen pretty much instantly, but the last little bit, when the seal has very little compression left, should take longer to force the fluid back along the pipes. And the extra pipe length, and associated extra resistance, on the LH side could explain atge small difference you report.

Paul Walbran

Paul
Thank you, that all makes sense.
Les
Les Robinson

Another question, sorry!
On the callipers, the pistons have a cutout/recess - what should the orientation of this be - top, bottom, front or back? Also what should the orientation of the shim be, big cut out top or bottom.
Thanks
Les
Les Robinson

Yes, another question!!
What brake pads are best? I currently have Yellow Stuff pads and I find that these fade if do lots of downhill braking and yesterday while on a 100 mile run made loads of dust on my wheels.
Les Robinson

Update

Using the TRW master cylinder my front (disc) brakes were binding after initial application - squeaking like made at low speed and creating loads of dust.
Didn’t know whether the callipers had started to stick. Anyway I decided to buy an AP (Lockheed?) master cylinder. These were not in stock when I got the TRW. Anyway the front wheels are really free to rotate after applying the brakes and not binding - not driven car yet though. TRW residual pressure?

The clutch on the other hand. If I leave it for a while then engagement is very close to the floor. After a few pumps the engagement is higher up. I have bled the clutch, must be a dozen times and no air comes out at all. Done it with the slave pushrod fully depressed, held with a tie wrap and also with it attached to the clutch fork. Used an Eezibleed.
Could this be an issue with the master cylinder?

Les Robinson

Bled the clutch the old fashioned way, with someone (granddaughter) pushing the pedal as I opened the bleed valve. Seems a lot better now.
Been for a short drive and all seems ok.
I was panicking a bit as a few of us are off to France on Wednesday so needed to get it sorted.
Spoke to AH Spares, where I got the TRW from. They had not heard of any issues with them but said if I send it back then should get a refund.
Les Robinson

If you want an AP Caparo (now ‘920 Engineering’, was AP, AP Lockheed, Lockheed Girling, Girling) manufactured 3/4 inch dual bore combined clutch and brake master cylinder - Unipart Part Number GMC112 - David Manners is worth trying to see they have in stock as well as Power Track Braking (Paul Hunt). I got mine from David Manners a few years back.

See: https://abingdonmgparts.co.uk/ap-twin-bore-mast-cylinder-3a-s-26m-62-63-mga-1500-55-62-disc-brakes-gmc112

Cheers
Mike
M Wood

This thread was discussed between 24/03/2026 and 21/05/2026

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