The first bump you hit the ball will probably jump off the seat and release any vacuum.
Easy enough to install breathers. You can buy some expensive kits online with 'billet' anodised aluminium blocks or you can use a few low cost pneumatic fittings and make a small bracket yourself.
I've advertised these kits in the past (I work in the pneumatics industry) but had little interest plus it caused a bit of hassle at work because someone had to take the parcel the to the Post Office (we use freight companies normally).
The fitting into the axle is 1/8" BSP. An all metal swivel elbow needs to be fitted then pipe both axles via a Y-connector to a female fitting and a sintered silencer (keeps the dust out).
Colin

