xk120 dual master cylinder

No, I wouldn't do that, alcohol is very corrosive.
BTW, I've run silicon in two MGB's, two Bugeyes, and my '69 E-Type and have never had any problems. My present MGB is my dailey driver as was its predecessor and I run it hard. After rebuilding my first set of LBC brakes and seeing all the corrosion and standing water in the cylinders I became a silicon believer. I've heard horror stories, but think its something other than the silicon - poor parts or poor method. I just bled my E-Type for the first time in ten years (longer than I would have liked) and everything was still nice and clean without a trace of water. DOT 3 & 4 are hydroscopic you know - they absorb water. I always soaked my new parts in silicon before installing and make sure things are very clean. I also fill the MC on the bench.
Stew Cleave
JOCO Chief Judge
'69 E-Type 2+2
and other LBC's

Well, I went out and got some Castrol LMA. Do you think I can just pump some alcohol thru the whole system and then pour in the Castrol?

Dick,

Yeah, I'm following the shop manual...my wife has spent at least 2 hours pumping the pedal. It's also important to open the bleed valves a full turn to get that flow. One thing I've noticed is that the front brakes expel about half what the rear ones do. Of course, the front (secondary) piston floats.

I think I will run that dot4 idea past the guy at the sports car garage. The pistons make a squeaking noise as the pedal is depressed. I am not happy having already changed it out twice, made doubly difficult because the nice nut plate for the m/c is missing.

John,
Before you go to any extreme measures, I would recommend that you disassemble the master cyl. rinse all of the rubber in alcohol, let it dry
thoroughly and then reassemble it using Dot 4. During one of my brake flails, I went from silicone to Dot 3 and the problems persisted.

The tilt valves are there to keep the system topped up when the pedal is released. When the pedal is depressed, the valves should
close tightly. You should be able to look in the reservoir and see but a very slight increase in the fluid level as the pedal is depressed
until the valves close. If the fluid level continues to rise, the tilt valve(s) are leaking. One of the toughest problems to overcome during
bleeding is getting a high enough flow rate to push the air past the high points in the plumbing.

To accomplish that high flow rate (and In case you haven't read it recently) note that the Service Manual recommends (Page L41)
bleeding both a front and a rear wheel cylinder at the same time.

Good luck (Dot 4 ONLY!!),

Dick Cavicke

I forgot to add in my last post that maybe the problem is due to bogus parts. M/c kits seem to come in baggies now. Does anyone know of a source for genuine Lockheed m/c kits?

I am using good old dot3 Prestone, the same as I've used in my E-type since I bought it in 1969. Interestingly, I've used silicone in my XK120 since it came out years ago. I rebuilt a resleeved m/c for it with a Lookheed kit, and it's been working ever since.

The current m/c is in another car. White Post does not state what fluid to use, but when I called yesterday they did ask if I were using silicone. That is not the case, and does not explain why the top outlet adapter and both tilt valves are seeping in a newly rebuilt m/c. White Post stated they test with air pressure only, and do not pressurize with brake fluid. There is a possibility that they rebuild the cylinders with silicone rebuild oil which holds air pressure, but fails to hold pressure when washed away by brake fluid.

I have been somewhat vindicated by visiting a highly regarded local sports car facility (XKD 515 is just inside the door) that is currently having the exact same problem with a 120. They have a so-called new m/c from a well known CA Jag parts dealer that will not firm up the pedal. They had previously tried a rebuilt one from another supplier that wouldn't hold pressure, had stripped it down and measured the bore and seals...everything appeared to be right, but that, of course, didn't help.

Bob may be on to something about the little rubber valves/cups. I just don't see how these valves would prevent the brakes from building up pressure. It looks like they are there to relieve pressure when the wheel cylinder pistons retract.

Ahhh, the dual master cylinder - I've had some Englishmen tell me to just give it the old Welly Toss. Dick's advice regarding fluids is good - if you use a fluid that swells any of the seals you will have trouble with the whole system. I've got a 140 DHC on DOT 5 with no problems, so its probably not a factor with the wheel cylinders. I have, had exactly the same problems you are describing; i.e., freshly sleeved MC, new rubbers, new valves and new tilt valves and the cylinder would not take a prime nor give any pedal. The problem here turned out to be faulty non-return valves - the fluid was just regurgiatating around itself. It took a couple of different vendors before I got a working set. I can't tell you if it was the fluid or not, just too long ago. I replaced the dual cylinder with a Tilton dual rig and adapted it to the 120's linkage. Did that in '90 and it's still in service, on DOT 3 high temp fluid.
Bob Grossman

John,
Are you by any chance using silicone fluid? If so, there's a strong likelihood that the pistons in the master cylinder and/or at the wheels are
neither moving nor retracting fully as a result of their rubber seals swelling.

If there are some rubber seals in your system, you're obliged to use Dot 4 fluid. I tried silicone fluid and ended up with a nightmare.
Everthing had to be flushed with alcohol, the master cylinder seals replaced (again) and everything started over with (Castrol) Dot 4.

I have had overall good luck with White Post. Once I had a wheel cylinder leak and they promptly fixed it.

If you need some additional suggestions, please contact me directly.

Good luck,
Dick Cavicke