Just replacing my master cylinder

i bought the JB1196 (PBR Model) to suit 120y disc front drum rear with no booster (11/16th size)

I have 4 wheels discs though (hilux 4pot front, bluebird disc rear)

i knew about removing the check valve, so i pulled apart the two bottom sections of the cylinder with the intention or removing the check valve from the rear brake circuit (to suit rear disc)

out of curiosity i also opened the front circuit, and was surprised to see a check valve in there

can any one explain?

ATM im going to push on just removing the check valve from the rear circuit as has been advised but im a little confused by my finding

Apparently Datsun fitted some a very low pressure check valve. It is common.

Have a look at the valve
If its the 'duckbill' style you should be able to tell the difference between them by looking at the sides

The drum one will be round... while the disc one will look similar but have 2 thin grooves down the side
These allow the fluid to drain back

Essentially a disc valve lets more fluid back than a drum valve.... some masters have no valves...some incorporate disc valves rather than none at all... makes them alot easier to bleed up when theres still about 2lbs of line pressure

the disc drum one i pulled apart had the same check valves front and rear and i ran it with disc disc setup with both check valves in with no problems. brakes never dragged

but why did you buy an 11/16th master to run with 4 piston fronts and disc rears....why not go something a little bigger. from what i remember the larger diameter ones weren't any more expensive

Small bore = higher pressure which is preferrable with no booster

yeah bigger master = more force to press and less travel

i'm happy with my pedal force/feel and there is not excessive travel, to me a bigger master would just necessitate the use of a booster

i'm spewin though, i got stuck out on the road...
i have been having trouble with brakes rubbing or holding pressure, they are heating up quite quickly and locking on

it seemed like one rear caliper a couple of weeks ago, so i figured it was binding, i had the rear calipers rebuilt and discs machined

problem still there so..

i dropped it into a brake place and they checked it over and suggested it needed a new master, a put that in today and problem is still there


ANY SUGGESTIONS?

i got it home today by releasing the bleed nipple on the master, this dropped pressure out of the system and would allow me to get about 1km, did it twice on the way home.

the pedal goes extremely stiff when the problem is occuring

at first seemed like one brake, rebuilt the backs and it seemed like both backs, but today with new master it seemed like all 4 holding

weird!

i had a lot of trouble bleeding the fronts, tried a lot with vacuum bleeder but it seemed to be pulling fluid really slow, then the only way i could get good firm pedal was to manually bleed the front (5 pumps then release nipple... over and over)

thats cool, i only ever ran mine with the 7/8ths. but figured there would be way to much pedal travel with that small a bore master cylinder. think i would have gone 13/16th on my landcruiser/r31 setup as the 7/8ths was a tad too hard for on the street

if you are having troubles with it grabbing then its most likely that either the rubber lines are #OOPS#ed or that the check valves are actually holding pressure

i can only speak from my own experience but like i said ive ran a disc/drum master on disc/disc setup with no troubles and had pulled a disc/drum master apart to have a look at the check valves and both had a groove down the side of them

yeah, im not really suspecting check valves here, as i'm having the same problem as before the master change

been running these brakes for about 5years+ so yeah.... im suspecting possibly a hose or that bias block on the firewall or a kinked line or ...... dunno blocked somewhere

will have a look at check valves in old master tomorrow, and look for these grooves, spring tension in front and back check valves felt very similar on new master

is odd that you are getting the same problem front and rear as they are completely different circuits.

what brake fluid are you using? you haven't used DOT5 or something have you. as ive heard that kills normal rubber lines

so the backs blead fine and just the fronts that didnt? but all 4 are dragging?