Air over Hydraulic trailer brakes... how to bleed?

G'day blokes,

Our old farm trailer has air over hydraulics on the rear axle only which seemed to have worked well enough until now. A couple days ago it popped a flexible line and having sourced a replacement they seem to be having problems getting it bled.

I figured it was just a matter of hooking up the truck as usual and working the control lever up & down, in conjunction with the bleed nipples, to pump fluid through the system like you might with a car.

I'm not sure if the master has now had a hissy fit so I have to ask... any special instructions?

Cheers for your help.

AB

iE
Gudday M8

Not sure wot V: of Air ovr Hydralic U got
Can remember th@ ifn they have run dry
U need to fill the resevoir n reach in the resevoir with a screw driver to toggle a little valve @ the bottom of resevoir where the outlet is to work the brakes

Then try bleed n them brakes again

themz the breaks

LOL
Cya
[ch9787]

Probably an air lock. Try connecting a supply of fluid to the open bleed nipple and push it (by gravity or air pressure etc) backwards to the master cylinder. Some garages will have a drum with a hand pump purpose made for the job.

Two things, the Master cylinder has emptied of fluid and the air left in there has created an air lock and every time you try to apply the brakes the master cylinder compresses the air in the line but does not overcome the resistance of pushing the fluid down to the wheels. Also make sure the bleeders are not blocked. Moving the little tippler valve in the reservoir will help to get fluid into the master cylinder but if the air is not bled out of the master you may as well get out that old barbed wire canoe with no paddles.

Pressure bleeding is good, i have used a windsreen washer bottle with integral pump connected to the open bleeding nipple, at the wheel cylinder it will push the fluid and all the air to the top of the system reservoir. Air is lighter than the fluid and will rise above the fluid first. If the master has a tippler valve it may pay to either take it out and refit later or push it open to allow maximum flow.
Much the same as in giving a medical injection a Doctor will remove the air in the needle before giving an injection, sometimes flicking the syringe to help the air rise.

Make sure the system is empty of fluid before starting though. Sometimes this is the only way to bleed Volvo clutches especially rear engined coaches with 30 ft hyd lines.

Pressure bleeding seems to be on the way out these days. Vacuum bleeding seems more common now, the fluid is sucked out through the bleed nipple, with a hose clamped over the end.

Saves having to worry about adaptors to fit the reservoirs, esp with older vehicles with odd size threads.

The other possible problem is that the master cylinder piston is now stuck at full stroke due to corrosion, but that will stop any fluid from coming through! I have used a common 5L garden weed sprayer with some brake fluid in it to reverse bleed many clutches with success also. Good Luck!