What is wrong with my brakes on my Ford Cargo 0813 and can I fix it myself?

Hi, I've got a Ford Cargo 0813 horse lorry. We had to charge the battery today as hadn't used it for a while, then I took it out only to find the air brakes weren't working very well (very scarey at a round about!!) The pressure seems to be going up too high (to around 8 instead of 5/6ish) and then when I stopped and put the 'hand brake' on I couldn't move when releasing the hand brake untill I had pumped the brakes a few times. Similarly when moving, if I pumped the brakes which reduced the air pressure to 5 then the brakes worked for a bit, until the pressure built up again. Any ideas? any suggestions will be greatly appreciated.