XK150 Slave Cylinder not retracting fully pumping up and pops out

Hello chaps, I’m in the UK and have been getting my old 1957 XK150 FHC on the road after many years of ownership. I have spent days on this problem with no conclusion. Just a little background, the car has a complete new clutch assembly and thrust bearing. What has been happening is that once out for a run the clutch progressively starts to slip. The reason is that the clutch has been left slightly in a compressed position. by the slave cylinder not fully retracting. I’ve spent days trying to fix this but am out of ideas. If I really pump the clutch pedal quickly the slave cylinder pops out.
I’ve spent hours trying to diagnose the problem. Now the master cylinder rod is adjusted correctly with just a little clearance between the rod and piston. The slave rod is initially adjusted with 1/16" clearance and there’s a good spring on the lever to retract he piston. Lever taper pin is intact.
What I have observed is that after several pumps the slave rod clearance disappears and no amount of spring pressure will retract it. Only when I crack open the bleed nipple does it retract by itself. I’ve had the slave apart numerous times and its clean and piston fully moves within the cylinder.
So the net effect is that the slave cylinder is pumping itself up and causing the clutch slip.
The only way it retracts fully is when the bleed nipple is cracked open(with tube into a bleed jar) and it
happily retracts back importantly by itself.
The hose has no kinks and doesn’t seem blocked as the clutch does work. Is old though
The only thing I can now think of is either a new hose or some master cylinder problem.
Any suggestions appreciated. It seems to have been a common problem but no conclusive resolutions.
Adam, London(Also V12E owner-both 39 &42 years)

Adam there is much written on the various forums on the possible failure mode of the flex hoses (brake or clutch) whereby they can act as one way valves. I suggest you replace it and use a BF goodridge braided type. Your problem should go away.
Best regards
Phil Dobson
(in deepest darkest Surrey)

I agree with Phil, replace the flex hose which has deteriorated inside and not causing fluid to return to the master. However if you already have a new hose your issue is probably with the clutch master not letting fluid return.

You say the hose is old, so it could be deteriorated inside and have a blockage that acts one way and not the other.
I had the same symptoms on my brakes, they would hold for an hour and then release, and the problem turned out to be in the master cylinder. This is after I restored it with a new rebuilding kit and silicone fluid. The new rubber seal swelled from the silicone and was thus just slightly too long, and when the piston was retracted, the seal was still blocking the tiny return hole and preventing the fluid from returning. A similar thing happened with my XJ12. Silicone is forever banned from my garage.

Don’t think it is the hose since cracking the bleeder ‘fixes’ the problem. I would experiment with a too strong return spring and see what happens. I don’t know specifically about the 150 masters, but I assume they have a residual valve (just like the brake masters on 120/140 cars) so I would check out the master’s operation thoroughly.

Have you checked to see that the clutch rod to the slave leads fair…in other words in a straight line “drawn” from the slave through the rod to the actuating arm. If there is too much of an angle it could allow the piston in the slave to jam. You do not want the slave piston to travel to far toward the end of the slave. It will become unstable. Adjust the rod so that it is firm with the piston fully at the bottom of the slave.

Maybe I’m missing something, but the fact that cracking the bleeder (on the slave) fixes the problem (until the next time) seems entirely consistent to me with the problem being an internally perished flex hose. Rather than allowing the fluid to return to the master, the blockage in the flex hose retains fluid under pressure, thereby preventing the clutch from fully releasing. Cracking the bleeder releases the pressure, allowing the clutch to release until the next time you try to use it…


Oops, my alleged brain had the rubber hose on the “wrong” side…sorry. So maybe an easy fix for the OP IF just the hose is bad?

I have ordered new seals for both slave and master cylinders and hose. I
have also ordered an additional slave return spring and will have one
opposite the other to retract the piston evenly. All cylinders are now
cleaned out and look fine.I will inform status when completed.

I have now rebuilt the master and slave cylinder and replaced the hose.
Contortionism at its extreme. What was also aparant was that the SNG repair
kit for the master had two main seals whereas mine only had one installed.
There was a grove for it so I fitted it anyway.This would have been the one
closest to the rod and so can’t see how it would have affected my fault.I
made a new slave rod as I wanted it longer for a new spring bracket I made.
I had noticed that having only one spring was pulling the rod away from the
centre of the plunger cone. Two springs will keep it central when
retracting. I also made screwed spring tensioners as springs were auful to
install otherwise.After bleeding it seems to work nicely now and no amount
of pumping up stops the slave piston from retracting nicely home. Fault was
probably a combiation of hose and master as intimated. Road trials later
this week. I enclose a photo and yes I know it’s not original but I’m in a
different space and my objective is keep it cheap and functional. My engine
is out of a 3.8 Mk 10 that I pulled out of a breaker 40 years ago for £50
and brought home in the passage side of my old Mini with the seat
removed.Happy days. Will update after trials.

Great stuff. Please let us know if its ok after the test drive. I love the account of the mk 10 engine. As you say happy daze!

do you have any pictures of 3.8L being a mini passenger?

I have road tested the car and the clutch is as good as it will ever be. I
have found that it needs full pressing down to function and the clearance
at the slave cylinder needed to be minimal. You can check with a 1" AF
spanner on the clutch fork to check the clearance which in effect ensures
the thrust bearing is not touching the pressure plate otherwise it will
wear prematurely. I don’t think I can get it to work any better. This could
be because I have a 3.8 sprung unit out of an E type if I remember rightly.
Maybe the diaphragm clutches require less movement?

Regarding any pictures of the Jag engine in the passanger side of my old
1960 Mini,sadly not, But the breakers yard where I got the engine from was
used in the 1970’s film ‘The Sweeney’. This was on the site of the old
Fulham Gas works (London) that now is an exclusive area. That very crane
seen in the pictures was used to lift and swing it into the car. ‘Eric’ was
able to manipulate that crane with precision accuracy. Link below shows
many cars that today would be salvaged.


i like the sweeney connection, will have to watch it again.
i think the clutch should work ok if the master and slave pushrods are adjusted as you say. i have found the slave likes to retain air bubbles and the easy way to clear it is to pump backwards through the bleeder to the high point. but i haven’t tried it on rhd 150. iirc clutch should disengage at 1/2 - 2/3 pedal stroke.

Hi all I have a problem, iv changed the clutch on my 1999 landrover discovery v8, now I come to bleeding then clutch witch a new slave cylinder and every signal time I bleed it the seals pop off can anyone suggest what can be the problem at all as I’m really confused, can it be a mistake on my behalf or is it a faulty part, thanks brendan from Leicester

Welcome to the Jaguar forums Brendon, it sounds to me like you have a faulty slave cylinder, I take it you are meaning the dust boot keeps blowing off when you try to bleed the clutch?
If thats the case is there fluid coming out as well? This would indicate that the seal is allowing clutch fluid to leak past and fill the void between the back of the piston and the dust boot.

Yes the slave cylinder had popped out but not due to faulty seal. Although I did fix this a issue long time ago I seem to remember the Master cylinder needed finite adjustment as did the slave rod. The issue was the slave rod was not staying in the cone of the slave cylinder as the one sided return spring was forcing it to one side. Thus the rod length was made longer and forced the cylinder out. Pop,splurt. I made a bracket and fitted a spring on the opposite side so the rod stayed central. I also have a old style heavy duty purple spring clutch assembly (possibly 3.8 E Type or Mk 10 manual. Its a heavy one. Later Jaguar clutch designs brought the slave cylinder much closer to the centre,inside the bell housing and I see why.
As I’m probably one of the last people around with the old Moss box I can see why others have upgraded to more modern or later box such as 420 OD.
Personally I think the original design is very poor.

Your problem is with the master cylinder adjusting rod, not the slave.
Reduce the length of the master cyl adj.rod by undoing the locknut and screw the rod clockwise to reduce.
This will retract the piston slightly and uncover the return hole in the master cylinder and allow the built up pressure to return.
The return spring on the slave will not pull the cylinder to one side if it working properly, it will only do this if the cylinder is fully extended and ready to pop out.
I had the same issue with the slave cylinder popping out due to built up pressure, it took awhile to figure out but the above cured the problem.

I am sure you are right and I did also adjust the master cylinder rod to have some slack with piston fully retracted.And yes if no slack it pumped it up and popped the slave.
I still believe the design is flawed and slave rod jammed on wall of cone in piston.I did fix it must be two years ago and in my case the pedal needs to be fully pressed in for clutch to fully dissengage. I don’t know if this is common? I wonder if others are getting better experience with diaphram clutch? I also suspect many have abandoned this gearbox. Because my car is an over 40 years owned hack I avoid unnecessary upgrades as I hardly use it but don’t need to sell it. Keeps the brain active,amongst other old iron of mine.

Adam, check the master cylinder is 3/4" and not 5/8". I had a similar problem and ended up solving it with the original supplied 3/4" master cylinder and I replaced the flywheel as well. My flywheel was a lightened flywheel as fitted to my 150S and although only machined once in the last fifty years did not give me enough distance from the block to the flywheel face. I have had no problem since and have no problems with gear changes in the original Moss box. Previously I was also popping the piston from the slave cylinder. John