Trouble filling/bleeding brake system

BrianK · 2026-09-25T09:04:11-0600

Looking for ideas.... 1973 Norton Commando 850

Just rebuilt the front caliper (pistons were sticking) and reassembled. Having a devil of a time filling the system. With the reservoir full, bleed valve open and connected to a vacuun pump, both pumping the brake lever and creating vacuum with the pump will produce air bubbles out of the bleed valve - there is SOME fluid in the system, but the level in the reservoir isn't dropping at all.

I have never had luck reverse bleeding, and don't have the equipment to do so in any event.

Right now I've got the reservoir cap off and the brake lever tied back, hoping maybe gravity will help.

Any suggestions most welcome and appreciated!

Tx - B

kommando · 2026-09-25T09:14:20-0600

What pistons did you use, some have a totally flat reverse side that the fluid cannot get behind to push them out, OEM and well made replacements have a lowered flat section on the outer portion of the reverse side so the fluid can get behind.

Tying the lever back and positioning the handle bars so the master cylinder is at its highest and leaving overnight has solved bleeding issues.

BrianK · 2026-09-25T09:36:39-0600

Thanks guys. Using aftermarket stainless pistons, originally sourced from Fred Eaton (Old Britts). They're in there now, but the backs are basically flat. Wondering if maybe I inserted the inner piston too far and covered the fluid inlet? - darned if I know how I would get that sucker out again if so, so hope not.

As I think of it, though, if the inner piston was too far in, the brake isn't going to work very well (!), but that should stop me from getting fluid in and bleeding it, I should think.

BrianK · 2026-09-25T10:43:11-0600

"Shouldn't" stop me, I mean

EstuaryBoy · 2026-09-25T10:50:59-0600

'I have never had luck reverse bleeding, and don't have the equipment to do so in any event.'

I went to the local vets and got some large disposable syringes, add a small length of hose and... Voila!!

concours · 2026-09-25T11:10:03-0600

Vacuum bleeders draw air IN around the bleeder threads, AND past rubber seal lips.

Remove the caliper and pre-fill it.
Leave the metal fitting off, gravity bleed the line to start.

https://www.ecstuning.com/b-motive-parts/european-power-bleeder/motivepb/?gad_source=1&gad_campaignid=19434531800&gbraid=0AAAAAD-pFG6AHFirnjarhLHqTj4BaC0Qv&gclid=Cj0KCQjwt9jVBhDXARIsAFSP-6f1ExIXxgYWSS4sbMKWXY6zcoWdLeJFZPooaS6s7so61FsZMlUVSbUaAo6sEALw_wcB

L.A.B. · 2026-09-25T11:11:19-0600

If the inner piston is too far in then it can block off the feed port which is in the sidewall not at the rear.

BrianK · 2026-09-25T12:06:27-0600

Thanks folks. Les, if the inner piston IS in too far, any suggestions as to how to move it? Guess even using compressed air is going to be of limited use then (not that I have access to it).

Concours, gonna try that, thanks.

BrianK · 2026-09-25T12:09:36-0600

PS, Concours, I have one of those pressure bleeders but have never found an adapter that fits the Norton reservoir. If you can point me in the right direction that would be great!

Cuppy · 2026-09-25T13:16:53-0600

Remove the caliper and pre-fill it.
Leave the metal fitting off, gravity bleed the line to start.

This

BrianK · 2026-09-25T13:17:24-0600

AI has its uses.... looks like GIrling adapter 1124 is supposed to fit. Online reviews say it doesn't work well though. Concours, do you have one that works well for you (and if so, do you know what it is and where you got it)?

BrianK · 2026-09-25T13:18:47-0600

Les, it's certainly possible the inner piston is in too far, although I thought I was careful about that. But even if so, would that affect ability to bleed? I would think not (?), although obviously the brake isn't going to work very well if only one piston moves.

L.A.B. · 2026-09-25T13:32:12-0600

BrianK said:
But even if so, would that affect ability to bleed?

It could possibly do as I said the bleed port is at the side of the caliper so if the piston is pushed in to far it could blank off the port.

DennisMo · 2026-09-25T14:33:55-0600

Why not try the old method. With bleed screw open, pull brake lever slowly all the way, wait 10 seconds after it is completely pulled, then tighten up bleed screw while lever is still pulled all the way, then let brake lever return slowly, then open bleed screw and repeat the above as many times as needed.

(The 10 seconds wait is to let the pressurized air in the piston areas work its way slowly to the bleed screw should the pistons be flat against the back of bore.
Maybe even wait longer)

Dennis

concours · 2026-09-25T15:45:13-0600

BrianK said:
PS, Concours, I have one of those pressure bleeders but have never found an adapter that fits the Norton reservoir. If you can point me in the right direction that would be great!

I have a universal rubber cone that attaches with wing nuts, fits most everything

Trouble filling/bleeding brake system

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