Following a change of towel rad in bathroom the system (15mm copper throughout) is losing pressure. The new rad has been bled but more air is blked out almost daily. Unfortunately the bathroom floor (house empty at time) was replaced by bathroom fitter and vinyl laid before I could confirm no leaks at new rad Boiler ha s had a pressure test and no problem there but when reconnected to system pressure drops. No obvious leaks, even on ceiling under new rad, Fernox leak sealer added 3 day ago but problem still there. My one concern is the amount of topping up done daily to restore pressure with no visible sign of water anywhere plus the amount of air bleeding out of new rad. Any advice would be appreciated

Thanks for the reply
As said the boiler flow and return taps were shut and pressure stayed at 1.5 bar overnight. Is the PRV on the system side so that pressuriing the boiler would not be affected by a leaky valve. I dont yhink the previuos houseowner had and protector in the water as there seemed to be a lot of black gunge in the rad I drained off to get the leak sealer into the system. So maybe when the new rad was fitted some gunge had been disturbed and found its way to the PRV.
Will go back and check some more possibilities

Does anybody know of a reliable auto vent valve for radiators. I have fitted an Aladdin type - it worked once but not again. I am trying to purge the air from the sytem as well as turning off 1 radiator at a time to try and isolate the fault.
I has been suggested that changing the bleed and blanking valves had cleared the problem of air getting into the system.

Thanks for the reply. As said originally I have searched ( a lot of the pipework is surface mounted so can be seen) the system for leaks and cannot find one. There is no sign of water on ceilings or walls, the new rad has been checked - no leak there. The system loses pressure in about an hour so if there is a leak it would have produced a wet patch somewhere. The system is topped up daily and the amount of air that is vented is significant. I will keep trying various options and see what actuall fixes the problem.

Thanks everyone for the replies. Problem solved - there was a leak is the pipework - finally manifested itself last night when some water came through ceiling. Turned out to be a screw holding floorboad nicked a pipe causing ever so small drip. Plumber said it must have been there for some considerable time as screw was almost rusted away.