Late XK120 stuck brakes

I am presently road testing my freshly restored '54 XK120FHC. It uses the later dual brake circuit master cylinder.
While driving I first notice the car pulling to the left and after some time I noted the car would come to a holt at traffic lights without application of the brakes.
On getting it back home and raising the front and rear of the car I found I could not rotate any wheels by hand.
Assuming there was trapped pressure in the brake circuits I opened the right hand front brake bleed which did release some fluid and, along with the brakes cooling, they all came free.
Is there a fault in the master cylinder? or it is adjusted incorrectly?
Regards, Graham.

Make sure there is adequate free play in the rod that pushes into the rear of the master cylinder. I think the Service Manual calls for 1/32 inch.


Do you know what condition the tilt valves are in? Back in the mists of time shortly after I purchased my car in the early 1960’s the tip of one of the valves broke off therefore it was not functional, unfortunately I cannot recall what symptoms that produced! However, Mike’s point is a good one as I experienced exactly what you are describing following a dual cylinder rebuild. The solution was to adjust the push rod to allow for more free play.


also had that…yup, more free play…I don’t know what the expansion of brake fluid is as it heats…but heat did seem to play a role when mine did that…Free play fixed it. Nick

I hope the rod adjustment fixes it. If not, then yes non-release of pressure is the likely second possible cause.
I had this problem with mine after I rebuilt the system with all new seals and silicone DOT 5 brake fluid. Mine is an earlier car with the single system.
The silicone brake fluid caused the seal in the master cylinder to swell, and it was just enough to cover the pressure release hole and prevent the pressure from releasing. See Plate L16 in the Service Manual, and the cup labeled F and hole labeled X.
If this were to happen on a tandem car, I would expect it to be either a tilt valve or one of the 3 seals labeled G.

or 21, 22 & 25.

I checked yesterday and the pushrod does have clearance (when cold)
I will give it slightly more and have another run in the car and see how it goes.
It presently has Penrite Dot 4 fluid.
Regards, Graham

mine was ok cold…when hot…brakes were on…and it was clearance…if only one brake…suspect a brake line interior…if all then as Rob said. Sometimes…informative to drive a short distance.,without brake use, .take heat temp readings on each wheel. Do same with brake use.

Took the car for another run. Same issue with brakes locking on.
Backed the adjustment right out so it had ample clearance but the piston didn’t release the pressure. Had to crack the bleeder again.
I.m guessing the fact that there is plenty of clearance to the piston it is either not returning fully or the seal still blocks the return port with the piston fully retracted?


Is your master cylinder original or a Reproduction, or sometimes claimed NOS - just old stock of reproductions made many years ago.

There have been examples of reproductions having a satisfactory outside casting, but the internal cylinder not accurately made, so doesn’t relieve pressure when it should. It’s not an adjustment issue, but a faulty part that will never work properly.

One example I am aware of kept the new-old-stock/reproduction outer cast housing, but reused his original internals with new seals only added, and things then worked as it should.

Don’t know whether you have same problem or not, but worth considering if none of the other suggestions are working.


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I am about to remove the tandem master brake cylinder from the car for further investigation.
1st question. Should the brake switch for a RHD car be n top or on the end? Two books I have give the exact opposite answer. Which is correct?
Regards, Graham.


My 120, like yours, is a 1953 and it has the brake switch on the top although it is, however, LHD. Check the function of those two tilt valves when you get the cylinder apart.



Refer to my JAGUAR XK120 AUTHENTICITY REFERENCE GUIDE, and also the associated JAGUAR XK120 CONCOURS JUDGING GUIDE (JCNA), both of which I co-authored.

The Stop Light Switch for all Right-Hand-Drive XK120 with Dual Master Cylinder should be mounted on the TOP.

For LHD XK120s it is mounted on the FRONT

(This was conclusively established/agreed, after considerable peer group expert review during the endorsement of the JCNA Concours Judging Guide)

Which book says otherwise???

Roger Payne



Hi Roger.

Opposite description in Bernard Viart’s book page 113.

Chris’s reply has his on the top and is LHD.

BTW. I have found the issue with my master cylinder and locking brakes. I will reply to this separately with a photo.

Regards, Graham.


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On removing and disassembling the new master brake cylinder from my car I found both compensating (tilting) valve pins crushed and bent off centre.
I can imagine that this would have be done on installing the valves with the pistons in their fully retracted position so instead of the pins going down beside the operating face in the pistons were crushed on top.
Thankfully my freshly fully restored 120 is not upside down in a field


Glad to read that you sussed out the issue with your cylinder. Clearly someone did not follow the manual when assembling the unit!


attached is a factory photo of a LHD XK120 with tandem master cylinder and the stop light switch is mounted in the top, the RHD cars I believe were mounted in the front same as the earlier master cylinder on RHD cars.
Any of the books need to be read in conjunction with SPC see page 39 A B C and D and plate AR etc.
the vertical stop light switch just visible lower right

Graham was this a new unit or your original rebuilt.
If new it wouldn’t have been disassembled if secondhand obviously whoever sleeved it and rebuilt it wasn’t sure of what they were doing.
Interestingly his is why Jaguar discontinued the use due to not being able to get through to mechanics how to work on them a number of mentions in Service bulletins.
Even on units not ruined you will find the pins slightly bent maybe just in operation?

of course can’t say this is factory.,but photos of my two tandem M/Cyls. brake%20tandem%20mcyl%20old%20pic

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In your second photo, is the protective plate between the cylinder and the down pipes reversed? On mine it angles forward covering the front section of the master cylinder, which could account for the notch visible in the insulating plate two thirds of the way along.


possibly,.I will have a closer look on the car itself…when I obtained the car…there was no plate at all. I found one…and then also the Bassett insulator part. I may have installed it that way thinking the exhaust pipe heat was more in that location…than the front where the pipe slopes aft…and there is more cool air flow. BTW…I have always been curious about the heavy metal U bracket on the frame rail…that does nothing…?? Not seen…but there…is the chassis number on the frame top.,.thinly stamped in the area just fwd of the m/c. Nick