won't build vacuum when handle released

Hi I just recently bought this pump and for whatever reason it is able to pull vacuum if i hold the handle down but when i release the handle the vacuum restores to no-vacuum - not sure what the issue is - i have disassembled and cleaned the entire unit and no signs of brake fluid or debris. Seals all look new as the unit is brand new with the exception of 1 time use which at that time it worked great - but no brake fluid has been in the gun