Brake system issues on 420

I have a 1967 420 I recently acquired and doing a extensive tune up. I rebuilt the brake system along with rear suspension and many other items.
I’ve had issues with two new Lockheed master cylinders. Ended up putting my original piston assembly in the new housing to get it to work correctly. Front brakes bled fine. Rear brakes nearly impossible. I tried pump method, pressure bleeding and seemed to get the air out, but no brakes in the rear. The booster/servo was also rebuilt. I don’t seem to have much power assist when I tried moving the Jag in my long driveway. I checked the vacuum system and it operates as it should. I’m suspicious of the trap valve in the slave cylinder on the servo for the rear brakes not letting fluid through. I had a similar problem with the main master cylinder. I put the original plastic valve in that unit and it’s behaving well. Unfortunately I don’t have another. I plan on removing it from the rear circuit to see if the rear brakes will work that way. If not then it looks like the servo assembly comes back out…
Could the shuttle valve in the servo slave shift over cutting the rear system out during the bleeding process?

Sorry to be a big vague but I’ve read about this issue somewhere not too long ago and It dealt with the problem in some detail.

I suggest you search the forum (also try the Daimler forum,

as mine’s a Daimler & I look there too).

Thought I’d give an update. Removed trap valve and it made no difference. Removed servo assembly and disassembled it. Found a cup seal got damaged somehow. Also the main packing was weeping into the vacuum unit. Found casting is very rough in the bore opening at packing. This is a power tune replacement. I find the new parts offered nowadays are not much good.

Jagman , Its gtjoey…we went through this with a whole bunch of etypes.
Even though it might say girling on the box or cylinder, they are CHEAP CHINESE SWEAT SHOP KNOCK OFFS.
SNG as of today, supplies the real part. Now there was a HUGE batch 20 years ago from india that are still floating around where the rears would not even get fluid as the return valve was crush, in all of them.
I got the sng and it works fine, they claim theirs is the real deal as 10 of us bought them with no issues.
Repro parts for the most part are terrible.

Thanks for the information. I got my master from Welsh ent.

In this case only Sng trust me

I believe you. Thanks!

I have a 3.8S which my British car shop recently changed the booster on. This is a single circuit system which may be different than that on a 420. They fitted a booster similar to the one on the left in the photos and this replaced the one on the right. I now find the boost marginal and may have to get my original one rebuilt and reinstalled or get the rebuilt original one from SNG Barrett. Anyone have experience with the SNG rebuilt one?

Brake booster orig SNG Barrett

My servo is dual circuit. I wondered about boost as I think it’s smaller than what would have been installed originally. I did buy an use original unit off Ebay yesterday. Just to compare or for parts. If I’m lucky the bore is good and I could rebuild it…

Go Sng if they screw it up they stand behind it and ten of us haven’t had a problem in the last two years