ive sent 2 bond it guns back recently with problems, how long have you had the gun? the only thing thats worked for me before is soaking different areas of the gun in gun foam cleaner and poking around at the ball on the end where the can screws on
as said just buy tools as you need them, if its just diy dont go ott with the best makes as you wont be using them every day, get a smaller tham 1m level though imo
looks like 19mm upvc quadrant, should cut off with a stanley and fit new ones with silicione on adhesive. they are around £1.50 - £3.00 for a 5metre length. If you paint the brickwork theres no going back