Certainly good advice - but don't mess around borrowing one - just buy a new one. Also check the seat inside the cap.....You can pressure test a system by filling it up and the putting a footpump on the rad overflow and clamping the schrader shut over it - it's close enough to hold pressure.
Bring it up to 15 to 20 psi and leave it...see what happens.
LOL Classic.............
What's "LOL Classic"? - it works - it's an easy way of pressurizing a cooling system - moreover the cap has a NRV/vacuum breaker built in so it can be left under pressure.
Have you nothing better to add to the discussion? Nearly 3500 posts and this is the best comment you can make?
Please don't derail a thread where an OP is looking for advice, go play on the "my cooper brake pedal fixes all braking system problems" thread .
Whats classic is the crap your talking..........
To do this you'd need to fit a blanking cap to the rad..............
ffs you can't put pressure behind a seal and expect it to pressurise the cooling system thats on the otherside of the seal.........
Remind me again what did you claim your job was in another post, something to do with fluids/hydraulics ?
No you don't - I'm really struggling to work out if you're just looking to make silly comments or if you actually don't understand. Never mind.
A standard cap has an air admittance valve to stop a vacuum developing when the system cools. I did mention it in my post, please try to keep up, especially if you want to try and and appear clever.
I'm an engineer who specializes in heat transfer engineering - through software developing to commissioning high temp/ high pressure (600C/1500psi) turbine applicatons. I don'y however think it's necessary to worry about what I know, but rather what you clearly don't know 
Edited by Captain Mainwaring, 09 August 2013 - 05:53 AM.
