XK150 Brakes binding / Master cylinder detail question

(David) #1

I’ve read all the available posts regarding binding disc brakes but I do have a small question . I have an XK150 3.8l OTS. Had really bad binding at the front so changed the original calipers etc to Coopercraft replacements. Unfortunately no improvement as the binding is quite severe - very hard to turn the wheel by hand when jacked up.
This is my detail question however. The original workshop manual advises that the Master Cylinder piston / push rod should be adjusted by "slackening off the clevis locknut and screwing the clevis along the rod until free axial movement of 15 - 20 thou ( 0.38 - 0.51mm) is achieved. "
As the clevis needs to be reconnected to the rod I can’t see how the free play can be measured , as the brake pedal is now connected.Is it sufficient to attempt to get the free play ( which is required to allow the piston to release and therefore allow fluid back into the
supply line and hence allow the disc pads to release) by using the pedal stop instead ( or at least just to see if this fixes the binding?) Can the piston / rod actually come out of the master cylinder if the free play is too much? I have seen a number of previous replies to this issue advise that the rod should be able to retract “as much as possible”
Sorry if this is going over old ground but any suggestions would be gratefully received


(Graham Knight) #2

I’m sure someone can answer your questions on here better than I, however, Besides a potential MC problem, it could be weak flex hoses. My 150 would not release the front brakes without a quick tap on the brake pedal, I finally tracked it down to the brake servo which has an internal valve that was bad.

(David) #3

Thanks Graham;
I put new Goodridge flex hoses on along with the Coopercraft conversion . The brake servo was " completely overhauled" during the restoration , but I must admit that there seem to be fewer Firms / mechanics around who can understand 1950’s technology even though the servo is pretty agricultural. It is on my list of suspects but due to its location on the car , I am hoping to find the culprit before having to dismantle servo and reservoir.



(Phil.Dobson) #4

disconnect the brake pedal at the clevis pin. if the brakes free off then you need to try and set the free play as the manual shows. If you hold the brake pedal in the full retracted position you should be able to ‘rock’ the operating rod slightly in the master cylinder (see me waving my hands)

(Rob Reilly) #5

Could it be this seal #3 failing to open when the shaft is retracted?

(Morris Barnett) #6

David . I would remove the original servo and replace with the better more powerful unit which is available from the usual sources, price is very reasonable at around $250 usd rebuild kits for the originals are now very scarce and pricey, which you are already aware of and then someone has to rebuild it for you, probably cost you way more than $250. With regards to the master adjustment, make sure you have 1 inch free play at the foot pedal.

(David) #7

Thanks for the replies . I am on to this over the weekend and will report back as soon as I have some results.
I haven’t accessed Jaglovers for a long time - maybe 3 yrs or so , but am delighted to find the same quick , friendly and helpful responses from members.

XK150 3.8 OTS
E-Type series 2 1970
XJR 308 2001

(David) #8

Thanks for all the responses. I have disconnected the clevis and the brakes freed off as suggested by Phil. I reconnected and adjusted the free play to a bit less than the 1 inch advised by Morris Barnett rather than the 50 thou shown in the original manual as I’m not sure 50thou is adequate - the brake pedals on my other cars all have at least 1inch free play and there is still plenty of motion left available in the pedal. However I will monitor it .