XK120 brake bleeding torture

I’ve heard of that. Never seen one though.

Sometimes the wheel cylinder bleed screws can leak air via the threads. One trick to remove that problem is to put a donut of grease completely around the threads and using a finger to make sure the grease is pressed into exposed thread and covers all possible air entry through the threads.

If you do that make sure you use rubber grease. If regular grease gets sucked into the cylinders its the beginning of the end for the internal seals.

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The last time I replaced my wheel cylinders I tried cylinder bleed screws with one way check valves. These let me bleed by pumping the pedal, but as mentioned I had to seal the bleeder screw threads so the screws didn’t let air back into the cylinders.


But I would start with a low pressure system in the fluid reservoir if I the MC were replaced because of the amount of air in the system.

I’ve seen this thread before and wondered what all the fuss was about until I recently changed my master cylinder!
My friend and I spent ages using the traditional method (with the added precaution of a non-return valve on the bleed tube) but could still not get anything near solid at the pedal.

For what it’s worth here’s my two penn’orth.

  1. While bleeding the back brakes especially we noticed that you could only pump 4 times before reservoir was in danger of sucking in more air.
  2. We surmised that if there was a slug of air near the MC then doing 4 pumps, locking off and topping up, by the time we were ready to start pumping again that air bubble had probably migrated back to the highest point in the brake line.

What worked for us:-

  1. set up the bleed tube and jar with the end of the tube in some fluid (as usual).
  2. person A opens the nipple, and then starts pumping the brake without pause for 15 or 20 pumps while
  3. person B continually tops up the reservoir.
  4. person B then makes one last pump, holding it down while person A locks off the nipple.
    As usual, start with the wheel furthest* from the MC and work round until you have done them all.

Repeat the process a day or two after when any micro bubbles have had a chance to congregate!
I hope that works for you.

  • For a tandem cylinder you could do the front before the back but on each branch of the tandem (front & back) do the furthest first.
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Yes, my technique on any car is to just crack a bleed nipple without fully opening it, then give 5-6 rapid pumps, holding the last stroke down while the helper close the nipple. 2 or 3 cycles per wheel and you should be done.


I know we all have our individual techniques for bleeding brakes, but I was under the impression that rapid pedal pumps tends to encourage micro-bubbles causing additional frustration.

I’ve found the rapid pumping technique is a way to extract larger air bubbles trapped in elevated places in the hard pipes. Otherwise, they’ll move toward the bleed nipples but not reach them, and when you stop pumping they’ll move back to where they were. The technique seems to work especially well when pressure bleeding (I apply only 10 psi or so) but you have to keep close tabs on the reservoir level. As you say, we all seem to have our preferred methods but this one seems to work for me…

Another thing about XK brake lines is they’re 1/4” ID, as opposed to 3/16” in other cars. The larger ID makes removing air bubbles just a little more difficult.