XK120 Tandem Master Cylinder Bleeding Valve Operation and Tilt Valves

I am looking for a description of how the bleeding valves (metal hat with a rubber cup in them) work on the XK120 tandem master cylinder. I have the cross sectional view that Rob Reilly has provided in the past and it is quite helpful as an aid to understanding the operation but I would like to hear from any of you who have a full understanding of how they work.

Another question I have concerns the compensating valves (tilt valves). It appears that they are under full pressure at their seals when the brake pedal is depressed. Is that correct?

I am diagnosing failed tandem M/C and have it all apart. The bleeding valves each have a tiny hole in the top of the hat that lines up with a tiny hole in each of the cups. I’m thinking this was done by a DPO and have never seen this before. Any thoughts on this?

John Brady
Bedford MA


The photo below of a bleeding valve might tell you how it works. I understand (but might be wrong) that it’s in fact a one-way valve meaning fluid can flow in one direction only. When pressure is on the side of the rubber, the sides of the rubber cup will close the 4 holes in the metal holder. If the pressure has been relieved or if there’s even under-pressure, brake fluid can flow through the holes. I’m not aware of any “tiny” holes in the rubber cup.

The tilting valves are rather delicate, especially when they have been assembled in the wrong way. This will lead to bending the operating rod of the valve and it will become useless. The picture below shows the “inner” of the tilting valve including the valve (complete with rod) and its spring I understand that brake fluid from the reservoir can only flow to the MC when the piston is in a defined position and the rod is “tilted” upward.

I tried to repair my original Tandem MC and even ordered a new tilting valve (rather rare nowadays). The rubbers for the bleeding valves are still available. But after several attempts (without any improvement) I ordered a new Tandem MC which worked first time! Even bleeding went well. These new Tandems look “more or less” like the originals. See pics below with new and original.

Bob K.

The bleeding valve with a hole in the center of the metal cap may possibly be a later variation, I don’t know, but it seems like it would not work right with a hole there.

A hole in the rubber cup might be to allow air to escape and not be trapped in there, which might cause the cup to be dislodged.

How it works: When you step on the pedal and there is pressure in the master cylinder chamber, it is sufficient to open the bleeder valve, i.e. allow flow past it through the holes in the sides of the metal hat and past the skirt of the rubber cup. If there is air in the wheel cylinders there will be insufficient pressure on that side to open the bleeder valve when the pedal is released, thus making it easy for one person to bleed the system. If there is no air in the wheel cylinders, the pressure on that side will open the bleeder valve, i.e. push the rubber cup and metal hat away from the sealing washer at the end of the chamber, and allow fluid back into the reservoir when you let off the pedal.

The tilt valves are only tilted, i.e. opened, when the pistons in the master cylinder are fully retracted, thus allowing fluid to flow either up into the reservoir or down into the brake system. When you step on the pedal, moving the pistons, the tilt valves close and pressure is built in the master cylinder chamber.

Bob and Rob,

Thank you for your insight on the operation of the bleeding and tilt valves. I was pretty sure about the function of the tilt valves, but not so much on the bleeding valves.

I know they are called bleeding valves, but I could not put their actual operation together. I did figure out that they are bi-directional one way valves depending on whether the brake pedal is pushed or released. And I could see that they were not part of the normal braking process.

So, as the name implies, the bleeding valves are designed to aid in the bleeding process. The trapped air pressure before bleeding is lower than the hydraulic pressure and does not allow brake fluid back into the master cylinder until the system is bled and closed.

Very clever engineering by Lockheed. Thanks for the explanation. I finally understand after all these years!

The holes in the hats and cups on the master cylinder I am looking at must be a bodge by a previous owner. It would defeat the design of the bleeding valves and bypass the valve operation by letting brake fluid flow in and out directly. That would make bleeding much more difficult.

I will replace them with spare originals.

Here’s a couple pictures of them and normal ones.

John Brady
Bedford MA

Why did the Jaguar Service Manual not use the term: “bleeder valve”, instead of: “combined inlet and outlet valve”? Was “bleeder” strictly Lockheed’s nomenclature?
I wonder if there were earlier/other versions of tandem cylinders,without “bleeder valves”.


Lockheed made a distinction between “Barrel” type (with a single output line) and “Tandem” type master cylinders. There were many, many different "Barrel " type MCs but only a few different types of “Tandem” MCs in the 1950s.
I checked a few of them like the NL and NM range, to be followed by the NO range (which is the one used by Jaguar on the XK 120s).
All these “tandem” MCs used a “Valve Body” with a (rubber) “Valve Cup”. The latter (Lockheed part number 108.128) was always part of the Repair Kit that could be ordered for every Lockheed MC.

So Lockheed never mentioned the term “Bleeder” or “Bleeder Valve”, just “Valve”. consisting of a body and a rubber cup.

Bob K.