Thermostatic mixer shower weird fault

[RBSRonnie]

I recently had a problem with my Chrome antique Gummers 1500 shower because it was constantly dripping. I bought a seal service kit and followed a youtube video to service the cartridge. When I reinstalled the cartridge I found that the shower would come on for the first 15 degrees then the water would go off. I adjusted the temperature but this had no effect. I stripped it down again and tested the wax thermostat - it appears to work although there's not much movement in it (@2mm). I then bought a new cartridge (£120!) and fitted it. The problem is still there - as soon as I turn it on I get water for about 15 degrees then the water almost goes off for the rest of the turn (drips a little).
I'm now really fed up with this - this is the only shower in the house - hence the family haven't had a shower for weeks now. If someone, anyone, knows what this problem is I would appreciate help. Thank you.

 

[Newboy]

Do you mean 15 degrees of rotation of the flow control?

 

[jeff the gasman]

You have probably got an airlock or jammed external valve on the cold supply and the shower is failing safe.
Jeff

 

[RBSRonnie]

Thanks for the replies. I've had to bite the bullet though and buy a new shower. Giving the bathroom a refresh at the same time. Hopefully we'll be able to have a shower within the next week or so.