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MG TD TF 1500 - Brake master cylinder failure

Myself and a friend both have 1955 MG TF's, and both of us have fitted AP brake master cylinders purchased from Moss Europe (UK) in the last 18 months. Previously we both had original Lockheed masters. In the last 12 months my friend has twice had total failure of the master cylinder piston seals (the latest was last Sunday during a run) and I suffered the same in March 2015, also failed piston seals. We both noted that AP uses long pistons with different profile softer seals. Moss has stated that bleeding the brakes using positive pressure in excess of 5 psi will destroy the seals. When I fitted my new master cylinder in January 2014, we used the traditional approach (one person on pedal & one bleeding), but when bleeding my system in March using 20 psi the seals failed during the process. My friend has not used positive pressure at any stage. However, we cannot see how unequal pressure either side of a seal can occur, having regard to the design of the master cylinder and reservoir.

My questions are 1) is this a failure reported by other TD/TF owners in recent times, and 2) is Moss's psi claim correct - and why, and 3) how do we resolve the problem to prevent recurrence?

I note that the MG Octagon Car Club (UK) sells master cylinders of TRW manufacture and replacement seals from Nelson Stokes of Cornwall. MGOCC report no failures reported to them.

Naturally we are both extremely worried about this situation .

N D Wallace

additional comment to this.
I have just dismantled the M/C that has failed twice.
The first time the master cyl just lost pressure and would not pump up, it had no loss of fluid. All I found was that the pressure seal was much softer than I am used to. I rebuilt it with new seals and all seemed ok, then it failed again this time the rear seal dumped all the fluid. Took it apart and can find no obvious fault beside the seals being more floppy than I am used to.
While it was apart I took measurements and test fitted various parts.
The new AP cyl has the recouperation and atmos holes in the same relative position to the back washer and circlip as the OE Lockheed. There is 0.5mm difference between the recouperation hole and the foot valve position which may even present on the OE (no CNC then :-} ).
I am going to assemble it with OE piston and seals and put under test.
It is quite frightening to have had three total failures with new equipment. I have been working on T's for over 40 years, had brake leaks but never an unwarned failure.
Ray TF 2884
Ray Lee

Master cyl assembled with OE piston and seals.
Piston moves under 0.5mm before closing recouperation hole and squirts a small amount of fluid as normal.
Now has a dummy brake lever with 150 lbs pressure at pedal end similar to in use normal foot pressure. I will now leave on extended test, releasing occasionally to simulate use.
Ray Lee

This sounds like a serious issue and a recall should be in order. The pressure limit for bleeding sounds bogus to me. George
George Butz

Hi George.
When pressure bleeding, the main seal is under no pressure differential as the recouperation and atmos hole are either side of it. It does put the secondary seal under pressure but not enough to matter. On the M/C that failed twice this was bled the old fashioned way.
I have been in touch with a UK supplier who says that although they have never had a failure reported they think the AP cyl is not good enough quality and will not sell them. They supply Lucas/TRW cyls.
Ray Lee

The Hybrid AP/Lockheed M/C has been cycled about 200 times against a dummy load to simulate piston movement and has performed as normal. Now back under constant load for a few days.
The OE seal is much more substantial than the AP replacement.
Ray TF 2884
Ray Lee

I found the cause of the failure on the cylinder that went twice. The seating on the piston for the secondary seal had a sharp edge which had nicked the cup. This allowed the fluid to leak out.
This does not explain the two failures when braking effort was lost without fluid loss.
I am surprised these are the only reported cases.
Ray TF 2884
Ray Lee

Could you tell me more about how you pressure bleed a TD? I've been of the impression that pressure can't be applied to the fluid if the pedal is not pressed. TIA. Bud
Bud Krueger

Hi Bud,
with the master cylinder full of fluid you pressurise the reservoir. This pushes the fluid down the 1/8" hole in the floor of the chamber. This pushes the fluid through the series of holes in the piston which is forced off its seat allowing the fluid to open the valve in the foot valve and down to the slave cylinders. Some fluid goes down the recouperation hole but as this is only tiny it takes the easiest route.
Too much pressure will stress the secondary seal as this is only supposed to work at atmospheric pressure.
Hope this helps.
I never pressure bleed, I have a connector which fits the M/C filler with a pipe to a container of fluid, as fluid leaves the reservoir vacuum refills it so no air enters. With auto bleed hose on the nipples it is a one man job.
Ray TF 2884
Ray Lee

Thanks, Ray. I can't see how you can pressurize the reservoir with your self-replenisher attached. I tried to use a pressure system (modified a cap) that I used to use with an MGB. But it wouldn't work. Guess I need at the geometry again. I have self-bleeder nipples on my TD. Makes it a simple operation, but I envy your self-filling system. Bud
Bud Krueger

I just took my Lockheed off that I rebuilt because I couldn't vacuum bleed it. Though either a seal was stuck or I had put it together wrong. All seemed proper so I put it back together. I filled the MC on the bench, used a screwdriver to push the piston and held my finger over the outlet as I released the piston. After six cycles the pump
Was pushing fluid. Reinstalled the MC and manually bled the system with help from my neighbors at Evil Genius Racing. After tightening a few banjo leaks I now have a firm peddle. Now if I can get the tub re sprayed and get the tank back in place I hope to get the carbs dialed in and get her out for a maiden voyage sans interior. I'm sure dad will be happy to finally see it at the house
W. A. Chasser Jr

No Bud, I can't pressure bleed with the self fill fitted, but I can quickly flush the whole system without worrying about entraining air.
The foot valve at the bottom of the cylinder prevents any suck back. I may be trying to teach old dogs new tricks, the foot valve maintains a pressure of about 7 lbs/sq in which keeps the slave seals pressed to the cyl wall.
Ray TF 2884
Ray Lee

Having had no responses to this topic outlining other similar failures, and being worried about another failure of the AP master cylinder whilst out on the road, I have decided to take action.

Advice from several UK suppliers suggests that the modern AP cylinder may not be to the same manufacturing standards as earlier versions or the original Lockheed. A recommended alternative is the TRW cylinder as supplied by the UK MG Octagon Car Club and Power Track International, amongst others. It has also been suggested that the pressure system of bleeding the brakes may hasten piston seals failure, although in the 3 failures outlined earlier only one was using this method.

I have now installed the TRW master cylinder and bled the system using the old fashioned method of one person on the brake pedal and one at the bleed nipple. So far so good. Any further developments will be posted to this forum.
N D Wallace

On a new system, I always fill the system backwards, by pumping fluid in the farthest fitting first and working up to the shortest line. An expandable rubber plug with an overflow line in it drains all excess fluid from the master cylinder reservoir into a container. Works for me and no need for a second helper. Good way to do hydraulic clutch systems also, as in an MGB and my Ford Ranger! Grin. PJ
Paul S Jennings

This thread was discussed between 14/05/2015 and 30/05/2015

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