XK150 Brake servo / booster stopped working following fitting new calipers

XK
1

I recently fitted new brake calipers on the front of my early XK150. I took the chance to drain the brake fluid - repeatedly pushing on the brake pedal until the brake reservoir was empty, as just draining under gravity gave very little flow. I then fitted the new calipers, refilled the reservoir and bled the brakes in the normal way (wife pushing on the brake pedal method…) The brake servo / booster no longer works. It worked fine before. I have not touched anything else - not touched the servo, nor the servo hydraulics, nor the vacuum hoses.
It seems unlikely that the servo could just have happened to chose this moment to fail.
What could have happened, please?
Many thanks.
Tony

2

Possibly the new fluid has swelled the old seals.

3

Thanks Robin - But wouldn’t that make them work better?

My symptoms are that the car brakes as if there were no servo. The pedal is hard, and if pressed before starting the engine it does not drop a little. So the brakes work - they’re just not assisted.

4

No, swollen seals tend to stick in the bore.

5

Hi Tony,
Is there a bleed nipple at the brake booster, behind the vacuum tank?
Regards
John

6

If it’s that, Robin, then I need to replace them? Or might they just come good in time?

7

Thanks Ledo.
There may be - I’ve not looked. But if there were air in the circuit within the servo, wouldn’t that just give me a soft pedal that would “pump up”? Would it stop the power assistance ?

8

Replacements the only way to go.

9

What kind of brake fluid did you use?
I only use DOT 4, Castrol if you can find it.
Never DOT 3 or DOT 5, because they have ruined the seals on 3 of my British cars. When I tried DOT 5 in my XJ12, the seals in the calipers tightened up, so it behaved as if the brake servo was not working.

10

Thanks Rob : I used DOT 4, albeit not Castrol.

I think I’m going to leave things as they for a while, to see what happens. Despite the servo not working, the braking is actually still better than before, as the new calipers I fitted were 4 pot (CooperCraft), versus half-ceased original “round pads”. I had been expecting a major (night and day) improvement - which in fact I have got (since prior to the change with no servo the car would hardly brake at all…)

If things don’t improve I think I’ll buy a modern replacement servo rather than try to replace the seals in the existing servo (which is now approaching 62 years old…).

Any thoughts on best modern replacement? And on the ideal boosting ratio?

Many thanks
Tony