breaking

[Nfld Z71]

I have a 2001 Silverado z71 single cab,a couple of times this week when i pulled out of my drivewayand I first applied the break the break was half hard and would not stop the truck after a few pumps the breaks were back to normal and working fine,has anyone else had this problem,any help wpuld be appreciated .

Thanks

[WEFs99]

Possible leaking caliper in the rear. If it was in the front you would feel a pull left or right.