XK120 - front brakes lockup on rebuilt system


Presently in the same process of bleeding my Tandem brake system. I searched for all recommendations, remarks and new “tricks” on this forum and made a list (see below). In addition, in the XK 120, 140, 150 Owners Workshop Manual issued by Autobooks Ltd (revised edition of 1979) they refer to a way of bleeding of the Tandem Master Cylinder I haven;t seen before (but that can be my ignorance…).
It reads:
On models fitted with the later Lockheed type of brakes, there is a tandem master cylinder. It is therefore necessary to apply the foregoing bleeding procedures to two wheels at a time, one front and one back wheel. In this case it is necessary to check that both compartments of the fluid reservoir chamber are topped up throughout the operation. I don’t know whether this works.

Other recommendations I found:

  • After installing the master cylinder and filling the system with brake fluid, loosen the front banjo bolt. While loose, have your partner fully depress the brake pedal. While the brake pedal is down, tighten the banjo bolt (and clean-up the mess). This will push any remaining air out of the master cylinder and allow the front brakes to be easily bled. This technique is not necessary for the rear brakes since the banjo is located on top of the master cylinder and air seems to pump out fairly readily.

  • Another trick is to keep the glass brake bleed jar above the bleeders. Place the jar on a front sus-pension component. This allows the air to rise up and out of the clear brake bleed hose. Then it is not pulled back towards the bleeders between pedal strokes.

  • The Master Cylinder can draw air in past the rubber. This happens because the fluid delivery can’t keep up with the “need” as you pull back the pedal. The key is to push fluid out with a pressure stroke and then ease the pedal back, allowing the tilt valves in the Tandem Cylinder to let new fluid in. Pulling the pedal back or pumping rapidly doesn’t allow this.

  • Chasing leaks is another issue, the banjo bolts on the wheel cylinders all can leak, as well as the reservoir tank and hoses. Use new copper washers and re-tighten all banjos.

Hope one of these points will help you .

Bob K.


Thanks for that info. I think I will try the two wheels at a time as that is easy.
As per the front banjo bolt on the MC, that is not easy to get to! my MC was wet tested before I installed it so I assumed that would bleed it of any air.
Per the bleeding above the bleeders, I have speedbleeders so no air is entering after pushing fluid out. just in case I also used the old fashioned two person method as well.
There are no apparent leaks in the system and I have all new washers.

How much luck are you having when applying all these tricks? This really should not be this hard!

So much for bleeding front and back together - no difference. I have passed over a gallon of brake fluid through the system thus far!

FWIW, I have never had to bleed two wheels simultaneously, and have no problems getting all the air out doing one wheel at a time. What works best for me is to manually pump the brakes (fast down, slowly up), with the fluid receptacle oriented to keep fluid in the hose. I’ve also had good luck with a pressure bleeder.
Like you, I find it takes quite a bit of fluid (2 liters).

But, I’ve not had my tandem MC out for repairs either I I’ve not started with a system that is completely empty.

I also have speed-bleeders at the front, but find a little air can get past them if the threads don’t fit tight in the cylinders. I’ve had to apply a little extra thread sealant on occasion to deal with this.

Long shot: have you adjusted the rear brakes to take up the “slack”? I find that if the rear adjuster is backed off too far, it takes 2 pumps of the pedal to get a firm response.

John thanks for your input. I did think perhaps there was some air getting in through the threads so after filling the system using the speed bleeders I then recruited a friend to do the old 2 person method and exclude that as a possible issue - but to no avail. Yes the rear brakes have been adjusted and the front ones are self adjusted,


Given everything else you have done/checked/replaced this may be redundant, but are you getting enough travel on the push rod into the master cylinder? The only reason I ask is because many years ago in an effort to better accommodate my height I thought it would be a great idea to adjust the brake pedal on the shaft by filing a new groove. After a friend and I had spent a couple of weekends and passed massive amounts of fluid through the system in an unsuccessful attempt to get a pedal a neighbour casually observed that it was almost as if the push rod was not going far enough into the cylinder (!) After returning the brake pedal to it’s proper position on the shaft we succeeded in successfully bleeding the system.

In all my years of ownership I have never had to use the “bleed two wheels at once” method quoted by Bob K. above and I have had the master cylinder off several times for rebuilds and removed a pair of wheel cylinders for sleeving and so on. My wife or a friend in the cockpit pumping the pedal while I bled each wheel separately has always worked for me. Good luck!


Thanks Chris - yes the pushrod is adjusted and going the distance within the MC. I also bench tested it before re-installing. I am running out of options but still think that there must be air in the system somewhere. I make this observation because I can pump up the brakes so they all lock and if I keep pressure on the pedal nothing bleeds off until I let the pedal up then it goes to the floor again and fluid returns to the reservoir!

It’s me again. OK I have narrowed down the problem but still a mystery!
When I push on the pedal I can lock up three of the wheels. The passenger side front wheel remains free but if I push another 2 inches on the pedal I can then get it to lock also. The brake adjusters are working properly as they have engaged a number of teeth (2 more than the driver side) using rubber shims I was able to determine that both front drums have about the same clearance. In bleeding each of the front wheels with 3 pumps I am able to get the same amount of fluid out of either side. When I remove the drum I can see that both of the wheel cylinders move. Is it possible that the air is not expelling from the wheel cylinders when I bleed? Any and all suggestions are welcome. Tom


An update on my brakes. I still had problems with my original Lockheed tandem master cylinder, although I had honed, replaced all rubber seals and one tilting valve (the other one seemed OK and wasn’t bent), but in the end decided to buy a new replacement tandem cylinder from one of the “usual suppliers”. My XK 120 is still a “rolling chassis” meaning access to the tandem cylinder is so much better now.
I saw two “brands” available from various Jaguar parts suppliers here in Europe, but in the end these are all the same and made by TRW. I thought BMC 25 would be another version but that simply is the type number of TRW, placed on the other side of the cylinder housing. I assume BMC (in this case) stands for Brake Master Cylinder and 25 is their type number for this tandem cylinder.

I replaced the old Lockheed Tandem cylinder, meaning the system was still filled with DOT 4 brake fluid, apart from the air that came in when I disconnected the brake pipes of the master cylinder.
This is a one-to-one replacement and (main) dimensions are identical.

Had to connect the pedal again and I adjusted the pushrod having sufficient clearance (probably a bit too much at the moment).
I had of course to bleed the system again and started at the rear (because the primary piston is connected to the rear brakes). Keeping the tank full, it was a matter of several strokes,before all air was out. Pushing the pedal down and letting it come up slowly, showed the fluid level in the reservoir going down at the end of the the stroke (meaning the tilting valve is working). Same for the front brakes.
Much to my relieve, I noticed a “hard” pedal and when I rolled the chassis, it stopped immediately when I hit the brake pedal. I know that this is by no means the final verdict and further adjustment is required once I can drive the car. As an example: pumping the pedal twice showed that the brake pedal comes “higher”. but none of the brakes has yet been adjusted.
So this all seems now good enough to start placing the body on the chassis.

Bob K.

Looks like you have a good brake system and certainly easier to work on than mine!
I have ruled out the MC as that seems to working fine for me but good to know that we can still get new ones.


Tom, I keep coming back to the suspicion of gunk, crud or debris, if not in the tubing, then in the tees.

I can understand that observation but I have cleaned all the lines, tees, banjo bolts etc and ran compressed air through them all. Also i bled with 3 pedal strokes from the left and then the right side and the fluid level in the container was equal. I have also used a caliper to measure the movement of all the wheel cylinders and they are moving properly although the movement in total on the Drivers side is more - this may be due to being closer to the MC or just a manifestation of the problem. I checked and rechecked the auto adjusters and they are working properly and I have the same number of teeth engaged on each side. This remains a mystery to me!

I would open the bleeder screw on the front one that is giving you trouble.

Fluid should run out via gravity (on all cars that I worked on for a front)

If it does not, that to me indicates a problem somewhere in those lines, hoses or cylinder)

The good thing is its an easy test to do, not much pain or skinned knuckles !

Yes fluid does run out from gravity and I can also bleed it and get as much fluid out per pedal stoke as the other side.