Sticking front brakes

I am working on a XK120 which has had aftermarket front disc brakes, the newer 2 stage master cylinder and a vacuum assist installed in its past. The front brakes will stick as if you are still pressing the pedal, after you let up. It will eventually let loose and it seems fine until i hit the brakes again. When the car came to me the brake fluid reservoir was completely empty and the owner was complaining of the brakes acting up. So, I fixed all the leaks, rebuilt the front calipers and blead the entire system. But, it still sticks. Just both fronts. I notice the vacuum assist is only hooked up to the fronts. I have read that it could be air in the lines, or dirt in the master cylinder or it could be rod adjustment. I am thinking the master cylinder may be bad especially when i pull the boot off the back there is some fluid present. But could it be that i haven’t properly bled the master cylinder after it was completely drained? i think i may just replace the master since it appears to be leaking through the back where the rod goes. i’d hate for the same problem to continue after changing out the master. is there a trick to bleeding the master?

It seems like good braking systems are often ruined by bad re-engineering or non-engineering attempts to improve them with front disc conversions.
Can you post a picture of this newer 2 stage master cylinder and booster?
If it looks like this:

our first guess is going to be the compensating valves, also known as tilt valves, not opening because the secondary piston is not sliding as it should.
If its different it may be totally wrong size or something.
Then we’ll turn our attention to the booster and calipers.

1 Like

thanks for the reply and the diagrams- The brakes were upgraded many years ago with what i assume is a dunlop disc caliper conversion kit. The master cylinder appears to be the one that you are showing above. The vacuum tank has a line from the front of the master cylinder and a line out to the front brakes only. I will attempt to attach some photos of the system. I would prefer to purchase a reputable rebuilt master cylinder, if its decided that is the culprit.

Before you start on the master cylinder
check the reaction valve in the servo.
Check the saloon forum regards this problem.
Disconnect the vacuum line to the servo then apply the brakes and check
they “free off”.
If checking on the road be prepared for a heavy pedal and remember to block off the vacuum pipe.
Peter B.

Moss Motors XKs has the C6712 tandem master cylinder on sale through Nov 15 at $240.

Thanks Peter - I was also thinking this could be the issue since its only connected to the fronts. So, if I unhook the vacuum and the brakes work and let go as they should, then my problem is the booster, or some part of that entire system? I will then start investigating what makes them work, especially the reaction valve in the servo.
Also - I have read so many conflicting statements on what brake fluid can and can’t be used, I am confused on what i should be using. Could the issues i am having been caused from using the incorrect fluid?

I definitely had a problem with silicone DOT 5 causing swelling of the seals in my '51 XK120 master cylinder, brakes stuck on, not releasing for about an hour, but that was the earlier single system.
Silicone DOT 5 also swelled the seals in the calipers of my '74 XJ12, pistons wouldn’t move.
I converted them both back to Castrol DOT 4.
You can’t go wrong with Castrol DOT 4 or another reputable brand DOT 4.

1 Like

would my problem be just the front compensating valve, since there are only issues with the front brakes? Just wondering as I look at the diagram and try to figure out how this master cylinder works.

Doug, Rob has advised re brake fluid.
If brakes are OK, servo disconnected, then most likely its the reaction valve operating piston which is positioned re photo. The white plastic cap has to be removed then the metal disc it screws to. The reaction valve is activated by this small piston twhich sometimes sticks in its bore resulting in your prob.
If you could reposition the servo 180 degrees the job will be easier, and in the long term,
if the problem crops up again, not sutch a bind.
The reaction valve is housed under the small raised cap that is clipped onto the larger
plastic disc, be aware of a small spring that seats the valve.
Peter B

Thanks Peter - Does the reaction valve need replaced or just the small piston or does it just need cleaned and lubed?

If your servo is newish I doubt any component needs replacing, just a clean and free up.
I recall a poster applied rubber grease to the piston to solve this problem with his servo.
Have you determined if the problem is the servo yet ? If not a alternative check is to
unclip the plastic dome then apply finger pressure to the reaction valve which might
seat the valve by pushing the piston down it`s bore, but again be aware of the small spring
that is clipped to the valve.
Peter B.

havent had time to check it yet but i will take your information and try to fix this issue very soon - thanks again!

i was able to unhook the vacuum to the booster and the front brakes still stick. I have been trying to order a master cylinder but they seem to be hard to find. I was told by a couple of the online stores that they are on back order until February. Any ideas on how i could get a new or rebuilt one sooner?

Couple of thoughts. Could be a sticky tilting valve in the master. This would be my first vote. Or, the booster cylinder piston is not returning all the way. P.S. I have also had problems with silicone fluid in these systems. I have been using Prestone synthetic fluid because it is not an aggressive paint stripper if you spill it.
Richard C

i thought unhooking the vacuum to the booster would tell me whether that was the culprit? since the fronts still stick with no vacuum to the booster, shouldn’t that lead more to the front tilting valve? could i just remove the tilting valve from the master and verify if it needs replaced or fixed?

You can certainly remove the tilt valve and check it.
I would remove the whole master and rebuild it.
Rebuild kits and tilt valves available from Moss and XKs.

thanks again Rob - i was trying to eliminate other potential issues before i tackled the master. I prefer to replace with a new master so i don’t have issues later from an insufficient rebuild by me. i can surely replace the seals and such, but resleeving and verifying the inner workings of the tilt valves etc. is beyond my knowledge base.

Resleeving may or may not be necessary.
Another easy thing you can try is to pry the pads back in with a screwdriver, then see if they move out again when the pedal is pushed.

It would be easiest to check the tilting valve first.

The piston in the cylinder attached to the booster wouldn’t care if the vacuum hose was attached or not if the piston was sticking in its bore.


so it could still be the booster? i was under the impression that if the vac hose was disconnected the unit would not work as intended and the brakes would not stick when used. i will remove the servo piston in the booster and inspect, clean,lube, etc Then, if it still sticks, i assume that means it is definitely the master cylinder? If it is the master, i just want to replace the entire unit with new and not fool with rebuilding. but i’d like to have all the other possibilities resolved. I have rebuilt the front calipers and replaced all lines. so, i dont believe any of that is the culprit.