RTL10 rough terrain lift brakes

Just picked up a Pettibone RTL 10 rough terrain forklift, I ran one 25 years ago in the Corp, but I can't remember much about the brake system. She has hydraulic brakes. The brakes work fine for a short period of time, then they're just gone (no brake, nothing) Shut her dowm for a few hours, fire her back up and I have brakes. Any ideas?

Get the pubs don't make a move without them !

If by pubs you mean manuals, I have them. Not much in there about troubleshooting the brake system. It must be something simple, because the work perfectly, then nothing.

You don't by chance have the exhaust pipe close to the master cylinder or brake lines do you? Heat will boil the fluid and then you just pump air.

Dirt