Is there a correct way to install the plastic brake MC extension reservoir from minispares? Coz yesterday the brakes were perfect, I had to undo the unions on the brake lines from the MC (long story), retightened everything and installed the reservoir extension with the little rubber grommet its supplied with. Now there's no pedal despite any amount of bleeding.

Could the extension be letting air into the system (there's a negative pressure there when undoing the cap)? What are the teltale signs for other parts of the system letting air in (leaking brake fluid?)?

The only correct way to fit the extension is not to over tighten it and snap off the filler neck! Seriously, there is no way fitting the extension would make any diff with bleeding.

I would suggest - remove all 3 pipes from the brass 3-way union on the firewall. screw in bleed valves in all 3 outlets, then see if you can just bleed the MC using one of the valves obviously and see if you can get pedal pressure. This will eliminate the MC from the equation. Then reconnect the 3 pipes in turn and repeat bleeding process for each brake line. If you have a booster, you can do it more easily by substituting a bleed valve at the outlet and check for air in both the booster & MC.

process of elimination.

I assume you did not remove the MC or disturb anything else.

there must be air in the system somewhere

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1967 Cooper S - new front bearings to do.
1965 Cooper S shell - Slow progress. No time or money!
1966 Deluxe- next rustoration!
Mk 2 & XJ6 Jags. Less said the better.

If air was let into the MC - can it still be bled out through the wheel bleed nipples - or will I have to bleed the MC directly?

Yep. It is probably better to do it that way, plus less messy - you don't end up with brake fluid all over your engine bay

_________________
1967 Cooper S - new front bearings to do.
1965 Cooper S shell - Slow progress. No time or money!
1966 Deluxe- next rustoration!
Mk 2 & XJ6 Jags. Less said the better.

Did it both ways last night. First bled the cylinder then bled the brakes. Have given up on the fancy one man bleeding kit and did it all the old fashioned way. Have brakes again now.

Good work!

_________________
1967 Cooper S - new front bearings to do.
1965 Cooper S shell - Slow progress. No time or money!
1966 Deluxe- next rustoration!
Mk 2 & XJ6 Jags. Less said the better.