When the shower is turned on to hot setting the water just trickles out. When on cold or medium setting the water pressure is ok. The hotter you want the shower the more pressure it loses. Any ideas?

Ok, sounds like the filter in the hot water supply is blocked. You need to isolate the water supplies, remove the shower valve ,turn on the hot water to see if you have full flow going into the shower valve. If the flow is ok then you need to remove the cartrige in the shower valve and check the internal filters. If they are clear you need to purchase a replacement cartridge from Gainsboro. Hope this helps. Derek. Ok, sounds like the filter in the hot water supply is blocked. You need to isolate the water supplies, remove the shower valve ,turn on the hot water to see if you have full flow going into the shower valve. If the flow is ok then you need to remove the cartrige in the shower valve and check the internal filters. If they are clear you need to purchase a replacement cartridge from Gainsboro. Hope this helps. Derek.