XK120 brake bleeding torture

You can also cut a push bike inner tube,

use a hose clamp to attach one end to the resevoir top

seal of the other end

pump the valve with a bicycle hand pump

apart from bleeding each joint from the to down via gravity, I also do the fronts first

If fluid wont run out the fronts by gravity alone, something is wrong

Yeah, something is seriously wrong. When I open the front bleeders, just a trickle of fluid comes out. There are no leaks in the system so it seems the issue has to be the M/C. I’m ordering a new one today. Taking that M/C in and out is one of the most frustrating things I have ever done on a car. Thanks to everyone for all the tips.


I recognize that removing the tandem master cylinder on an XK is not the easiest chore (ever tried removing the door hinges?). However, when first removing mine I first drained the fluid reservoir and disconnected the rubber hoses connecting the feed pipes, then disconnected the brake line that feeds the rear and then the front. Yes, the two bolts that go into the tapped bar within the frame mean being bent double over the front wing, but removal is at least a bit easier than refitting. Lifting the unit out I disconnected the pipes that go to the tilt valve assemblies on the bench, mainly because they were seized and rotated the pipes when I tried to undo them. A propane torch later resolved that issue.

I think I rebuilt my original about 3 times before I got it sleeved internally with brass. I assumed that this would solve the problem as the original bore had gotten pitted and over time was damaging the internal rubbers, but the company who sleeved it neglected to bevel the edges of an internal orifice and, eventually, it started to eat rebuilt rubbers again. I finally opted for a replacement cylinder when the price of rebuilt kits got completely ridiculous. That unit, from one of the usual suppliers, has been on the car now for several years with (touch wood!) no issues.

On reflection, maybe it didn’t seem like the worse chore because I have ended up doing it so many times. As John F. of this parish wrote some time ago when he was having motor issues, he had taken the head off so many times he could not only do it in his sleep, but in record time to boot! I think you have narrowed it down to the cylinder. Don’t give up, think of all the fun you are going to have once the issue is resolved. I gave mine it’s inaugural run of the year yesterday in glorious sunshine and 19C temperatures. A good run more than compensates for all the frustrations over the past 58 years! Good luck.


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Thanks Chris, after a nights sleep, I realize it could be worse and after all I DO have a great car. I got the M/C out this morning and it was all put together according to the manual but I don’t remember the tilt valves having that small plunger that is spring loaded pushed to the side. I suppose it’s called a tilt valve because the valve plunger IS tilted to one side but not sure. Is it supposed to look like this? Pushing the little plunger over causes it to return to the side. Is this where it all went wrong? By the way, when I took the tilt valves off, the front part was full of fluid and the rear was empty wet but not filled like the front.

Here’s the pics of the tilt valves…


Sorry to answer a question put to someone else, but the first thing you should do is check whether the pin/needle in the left tilting valve is bent (or still straight). Many tilting valves have become defective due to an incorrect assembly whereby this needle is bent. See pic below for another example.


The needle is operated (pushed upward) by the piston inside the MC and when it is bent, it will not function in a proper way. New tilting valves are hard to get and if you find one, they’re expensive.

The subject has been dealt with on the forum several times.

Bob K.

Thanks, MOSS sells them for $75.00 a piece. Not too bad. I’m just going to get a new M/C for $320.00. In the pics you sent, the one on the left is like mine. So am I correct that by “bent” you mean like the one on the left is bent and defective? If so, then that is what the problem is with my M/C because both of mine are like the one on the left…

I learned from other members here, that you should install the tilting valves when the main pistons have been placed in the cylinder and these pistons should be pushed (and kept) inward before you tighten the tilting valves. Have a look what the “pro’s” have already shared on this forum.

Bob K.

Right, I get that but you didn’t answer my question…if the needles are over to the side does that mean they are “bent”? Should the needle be in the center? I am assuming that I put them in wrong and bent them so it’s safer to just buy a new M/C. Thanks for the info.

Yeah, the needle stems should be centered. They are made on a lathe so they would have to be symmetrical about the axis.
This cross section shows the tilt valves (compensating valves) in the tilted position, when the pistons are moved to the left, brake pedal off, no pressure. You can see the valve seats at the top are cocked over, letting fluid in from the reservoir.

When the brake pedal is pushed the pistons move to the right and the tilt valves go to the non-tilted position, straight up and down, closing off the reservoir.
The pressure in the two chambers opens the bleeder valves and fluid goes on to the wheel cylinders.

On page L.45 of the workshop manual, in describing one of the final stages in the reassembly of the tandem master cylinder, it states: “Insert a suitable rod in the rear end of the master cylinder, PUSH BOTH PISTONS FULLY FORWARD in the cylinder and screw home both tilting valve assemblies” As the words I have capitalized are in bold type in the manual clearly this is an important step in the reassembly process.

Indicative of my previous comments above, these pages of my workshop manual are amongst the most filthy and begrimed! You’re getting there!


Got the new M/C installed yesterday and spent several hours trying to bleed again. Fluid flows freely but I just can’t get the air out. I’ve tried wrapping the bleeder valves with teflon tape but still no luck. I am using a pneumatic bleeder with my air compressor at about 60 lbs. I’ve seen guys on the forum talking about pressure bleeding that requires a top for the reservoir but can’t find one. What am I missing with this bleeding issue?

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I really think you need to go back to the tried and true method of brake bleeding.

  1. Fill the reservoir can 2/3
  2. Find a piece of clear plastic tubing that will fit snugly on a bleeder nipple.
  3. Find a glass jar with a lid and drill a hole in the lid for the tubing. Fill the jar 1/3 with brake fluid.
  4. Obtain a brake bleeder wrench. Do not use s standard open end.
  5. Starting with one of the rear tires slide the wrench over the tubing and onto the bleeder nipple.
  6. Have an assistant push down on the brake pedal and hold it down.
  7. Loosen the nipple to release the pressure.
  8. With the assistant holding down on the pedal, tighten the nipple.
  9. Have the assistant release the pedal and push it down again.
  10. As before, unscrew the nipple to release the pressure and retighten.
  11. Continue this process until clear fluid is flowing thru the tube.
  12. Add fluid to the reservoir can.
  13. Repeat on the other wheels.
    Pat H

I’ve never used air pressure to bleed a system and can’t imagine why it would need 60 psi. I don’t understand what you are doing. Where are you connecting the air pressure if not at the top of the reservoir?

Im assuming by 60lb pnumatic that your sucking from an open bleed nipple…doing this method you wont touch your brake pedal. …and are keeping the fluid reservoir topped up…if you sucking in air then you should be able to find where the air is being sucked in from…maybee its time to disconnect front from rear brakes and block pipes and just try bleading the fronts…if fronts bleed ok then extend the lined and bleed rear…Steve

I’m in the process of bleeding my complete system as well (having the Tandem Lockheed Master Cylinder on my 1954 XK 120 OTS).

Found this instruction from Bristol on the internet that deals with the same tandem brake system (Lockheed MC 31526) but a slightly different bleeding procedure is recommended by Bristol (or was it Lockheed?).


To bleed the brakes in pairs is not new and seems also logical given the fact that we have a Tandem MC.


Although I vaguely remember that Rob R. mentioned this once, what is different is that they "open the bleeder one complete turn". And also new for me is that they don’t close the bleeder screw after every pumping action, but simply continue pumping slowly until no more air bubbles are seen (of course with the hose in a jar with sufficient brake fluid to prevent that air is sucked back in the system). One could do that without an assistant, apart from the very last action whereby the bleeder screw is secured with the pedal depressed.

I’ll have a try using this method, just to see whether it works.

Bob K.

I think I said a quarter turn, but anyway whether a quarter or a full turn, the theory is that the expelled fluid will fill up the bleeder nipple threads and not allow air to backflow in past them. Works for me, I never need a helper.

I’m trying to guess, are you using one of these things?

That is why I always use a second person to hold down the pedal until the bleed screw is tightened — to keep from sucking air thru the threads

Good idea. Will give that a try.

When I had a BMW, I had a hand pump tank bleeder specially made for BMWs. It and the car are long gone, but as I recall, it pulled a vacuum on the reservoir, which pulled all the air up to the top of the fluid level and out into the tank. You did not open the bleeder nipples on the calipers at all.