New battery in but doesnt hold its charge when left for 2days

Could this be an electrical fault somewhere? something shorting out?

changed the alternator brushes and when driving the gauge says 13 volts as if it is charging when the engine is on cf_emoticons__BangHead:HELP

It wouldnt be uncommon for your new battery to be defective. Saying that it sounds like theres a slow drain on it from somewhere. Have you got an alarm or anything like that fitted? Id try putting a multimeter accross the battery to get a more accurate reading. Try it with the engine running, then switch it off, see how quickly it drops back.
Chris

tried the battery with mine in my other mk1 golf and that does the same thing, possibly an alarm system i will see what i can find behind doorcards and under steering wheel
etc

Also with the meter connected and the engine off, try removing the fuses one at a time, that way you can see what's taking current and what's not. It cold still be a battery fault, some garages will do a 'heavy load' test for you and this will determine if its the battery immediately.

Put the multimeter between the battery + and the + cable then you can see what amps the car is drawing when its all turned off. It should be very low, remember to turn off the interior light if you open a door. Pull fuses until the drain drops, I would start with the radio/int light fuses first. 

cheers for the input guys, going to check this next thursday will let you know how it all goes