6x6 Brake Master Cylinder

[Summiitt]

One of my 6x6s lost all brakes yesterday morning after or during hitting a Kangaroo, the guy driving it made it back to the workshop and got another ute to get up into the bush.
I initially assumed the roo had taken out a brake line, however when I looked at it, the front pot in the reservoir was empty, the rear was full?
There are no obvious signs of leaks anywhere on the system and all lines are in place.
Anyone here know whats the idea of the individual pots in the master cylinder?
Im going to refill and bleed the system and see what happens..not a confidence inspiring fault..

[123rover50]

On mine the big rear pot does the rear brakes and the smaller front pot the front.
I thought it was like a split system and if the front went you would still have brakes at the rear.
Seems not:(
Dave will be along to tell us.

[Silenceisgolden]

On my 6x6 the master cylinder emptied into the brake booster. I had a new master cylinder fitted, but no-one - not even KLR - can bleed it to get a good pedal. Someone on the forum suggested that the master cylinder should have been 'bench bled' before fitting, but I am not sure what this means.

[Blknight.aus]

the failure of one side is supposed to allow the other to operate.

bench bleeding is done to make the bleeding process quicker and easier as there are 2 semi independent pistons that have fluid between them, if you bleed up using the mechanical push partsonly without the fluid working you wont get a good bleed. by bleeding by hand prior to installation and getting the fluid running properly you almost always get the bleed right.

it is possible to pump the front reservoir into the rear one if an internal seal has failed and its possible to pump the mc into the booster and not know because the vac pump takes it all and puts it in the sump.

theres 2 reasons you cant get a good pedal its always one or both of these.

1. the booster is not doing business properly, you'll pump up the brakes manually and then it goes all squooshy when driving, this is typically the balance part of the valving for the booster failed.

2. the middle part of the mc is not filled correctly and has air trapped in it along with excess pad or shoe clearance.


I like to bleed by the bench bleed to make sure everythign is ok (and if you have the MC with the differential switch make sure its in properly) then check the pedal action by bleeding the MC at the delivery ports. Then bleed the fronts and then bleed the rears. bleed at the T joints first and then do the individual wheels.

[123rover50]

On mine the big rear pot does the rear brakes and the smaller front pot the front.
I thought it was like a split system and if the front went you would still have brakes at the rear.
Seems not:(
Dave will be along to tell us.

I should say on mine I could not fit the integral booster so I just bolted the master direct to the pedal tower and fitted remote boosters, one for each axle, near the axle.

[Silenceisgolden]

.

bench bleeding is done to make the bleeding process quicker and easier as there are 2 semi independent pistons that have fluid between them, if you bleed up using the mechanical push partsonly without the fluid working you wont get a good bleed. by bleeding by hand prior to installation and getting the fluid running properly you almost always get the bleed right.


1. the booster is not doing business properly, you'll pump up the brakes manually and then it goes all squooshy when driving, this is typically the balance part of the valving for the booster failed.

Thanks Dave,
My brakes were fine until the master cylinder leaked fluid into the booster. I topped up the fluid, and the brakes were fine. I took it to a Perentie mechanic to have a new master cylinder fitted, but he was unable to get a good pedal with the new mc.


.

2. the middle part of the mc is not filled correctly and has air trapped in it along with excess pad or shoe clearance.

Is there a way for me to get rid of the trapped air without removing the mc? Thanks for the advice,

Cheers,
Paul

[Red90]

Sure. You can “bench” bleed on the vehicle. Disconnect the lines and make up some to recirc back to the reservoir.

[Silenceisgolden]

Sure. You can “bench” bleed on the vehicle. Disconnect the lines and make up some to recirc back to the reservoir.

Thanks Red90, I will give that a go. Cheers, Paul

[Red90]

https://www.onallcylinders.com/wp-content/uploads/2018/07/10/Raybestos-Master-Cylinder-Bench-Bleeding-Video-Still.jpg

[Blknight.aus]

sometimes.....

you have to tilt the MC to get it to bleed up properly....

bout 25-30 degrees from memory

you'll hand pump it level, then turn it sideways a little give it one or 2 pumps and another couple of shots of air will come out.

I dont know why, although the drillings look like they are in the sides of the cylinder they actually come out really close to being at the top of the bore internally. My pet theory (and this means I'm guessing here) is that the brake fluid builds up pressure around the port and the surface tension creates a temporary seal that then traps the air pocket but the fuild can still get through as its not limited by the surface tension. When you kick it over the bubble moves and gets caught in the main flow.

The Trick for the SII Clutch Master of jacking the front of the vehicle so the MC is oriented with the front above the horizontal plane also seems to help.

Me? If I was you the first thing I'd try is to remove the bonnet, get the front left as high as possible and see if it wont bleed up on the vehicle using a couple of bleed nipples with the tip ground off and some tubing as shown in the photo linked above.

[Summiitt]

Finally got round to sorting the lack of brakes and disappearing brake fluid on the 6x6..turns out the front left caliber is weaping fluid and has done so for some time, unbeknown to me!
The guy who drove it for the past 50k forgot to mention to the new driver that he tops up the front pot in the MC every couple of weeks..not really any visible signs of leakage as it had massed into a greasy clump of dust and mud.
Ill install a kit in the caliber tomorrow night and all should be good.