Et, only single cyl with all the machines in question. Thanks for all the replys. I am going the let these problems die of old age as the machines are starting to be considered for trade. That single glitch with the ys 80s left a poor taste in the end users mouth, certainly encouraged by the annoyed operators. Ruben

2 master cylinders in this truck, one on the right is brakes only one on the left is inching, both master cylinders need free play on the shaft, depending on the size of your truck, you may also have a bad "crossover" valve.

Bbforks no hyd boost system. Must be later ser#where they did away with mico-lock cyls. Clark couldn't catch a break on that one. Can't empty all dot 4 with out pulling finals and brake pistons.ruben

like brewski said do not mix any of them
if the system has DOT3 use DOT3 only

same with DOT 4 or 5
none of these intermix with each other very well and particularly the DOT5, it is synthetic so it will not mix with anything because of this.

And like brewski said if you want to change the fluid type you will need to clean out the complete system first before putting ANY of it in the system.

I haven't heard of DOT5.1 yet but it wouldn't surprise me if the engineers were coming up with a new fluid, they are good about that.

You should not mix either. If you switch to dot 5 you need to fully disassemble the brake system and start out with new rubber parts. Dot 4 would be a better choice for higher temps in a dot 3 system. There is a new fluid Dot 5.1 that is compatible with all fluids. It is pretty hard to find.

I have heard you can use Dot 5 in Dot 3 systems but you can't use Dot 3 in Dot 5 systems ?? I think there was a Crown tech report on this.
Not !00% Anyone agree or disagree??

Have you checked the hydro-vac? I've serviced many a C500-80 with that complex brake set-up with the hydro-vac. I've had that booster do all kinds of weird things from not releasing the brakes to intermittently not working at all.

I don't think silicone brake fluid will make a difference unless you find the brake fluid boiling as this is the only advantage it has- ( Higher boiling point).

Breski lots of free play. 1/2 the pedal throw. Leave it overnight and you can use the lift normally for a 1/2hr. All the ys80s I servive suffer the same malidy, same cyl, fluid, and timeline. 4 different customers, all out door dusty yard work. I can't be the only one with this experience. On one truck I used a single volume cyl that went in easy w/mod to push rod. Worked ok but not enough to hold firm on slopes and up side of trailers iif divets on ground caused mast to bang on side of trailer with owners-ops protesting. Could not get single cyl with swaller bore w/same mounting configuration.. Ruben

When you say pedal play at it's max. Do you mean no free play or a lot of free play. you need about 1/2 inch free travel in pedal to allow brake fluid to return to the reservoir. Silicon fluid will not help if you have no free travel.

Brew, that has been addressed many cylinders ago. With the pedal play at its max, the cylinder still holds enough residual pressure to hold the inching cylinder engaged and in some circumstances enough pressure to hold fast on the brakes enough to stay midway on loading dock ramp until pressure bleeds off to roll free (90secs) and to relax the inching cylinder, at least that or longer. An eternity when an operator is trying to off load in street. The failure seems to be related to under seat temps. Going back through archive invoices, they seem the group around the warm months. Keeping the radiator core clean lenthens the useable lifespan only slightly. I have taken old ones apart and think I see some swelling of the rubber parts but without new parts to compare, hard to tell. Back to orig question, would silicon fluid improve performance? Rubin

Check for free travel on the inching master cylinder. Some times this gets adjusted too tight. This builds up pressure and causes the transmission to slip or not engage.

Does the truck have a micolock two stage mastercylinder? I service a few clarks with that unit that hold a residual pressure in the brake system that disables the truck a few minutes at random, than turns the truck unusable. I have one to be changed now. They seem to last a year or so, have been using dot4 fluid. Considering going to silicone fluid this time out. Any thoughts? Rubin

also look for fluid coming from this inching valve
there is weep hole in side of valve, if you have wetness around valve weep hole then it is leaking and should be resealed or replaced.
do the same for inching master cylinder, pull back boot at rod and see if fluid is present inside boot, if fluid wetness then master is leaking and should be resealed or master replaced.

and yes also check rod freeplay, as was stated, 1/8 inch freeplay at rod or 1/4 to 1/2 inch freeplay on pedal before rod touches piston.

The transmission control valve is on the left side of the transmission. From the factory there would have been a bleeder on the brake line fitting (1/4" wrench) going into the inching valve. When the machine stops pulling, pull the floor and crack the bleeder open. If it had pressure, it should begin to pull again. Chances are the inching master cylinder (left side) push rod is set too tight, needs 1/8" free travel.

it is a model c500 y45
serial no y555 0089saf 4323