Where is the master brake cylinder located on a 1988 mitsubishi L300 van?

Install new spark plugs, new spark plug wires if needed, a new gas filter, a new air filter if needed.

clutch slave cylinder or clutch master cylinder problem
if is more than 4 years since either was replaced , now would be a good time to do it

Look and see if there is a second master cylinder under the hood of your truck, near the brake master cylinder, in fornt of the driver's seat.
This would be the clutch master cylinder, and should be slightly smaller than the brake nmaster cylinder.
From your description, that clutch master cylinder may be out of fluid, brake fluid.
Probably your slave cylinder has failed, it is attached to your transmission on the other (lower end) of the brake line from the firewall master cylinder.
Adding brake fluid might get the clutch to work long enough to get your car to a garage or up on some ramps so you can fix it.
Be blessed

Yes. First, are there 2 master cylinders under the hood? If so, there is a slave cylinder by the transmission, under the car.
The larger master cylinder should be for your brakes, and it should have about maybe 4 or 6 brake lines attached to it.
The clutch master cylinder should usually have one brake line attached to it.
If you will follow that brake line down, you should find the slave cylinder at the other (lower) end of it.
As far as whether it needs changing, these two things will tell. Does it leak or stick?
From your description, sounds like it is sticking.
Be advised, you might also have a problem inside your transmission bell housing with the clutch, the throwout bearing, or something else.

Put morw brake fluid in clutch master cylinder, pump clutch paddle, look under car for leak, if no leak replace clutch master cylinder, not holding pressure.

The brake booster is a vacuum operated mechanism that is obviously failing on your van. It's the big round device that your master cylinder and bolted to (mastyer cylinder is directly under the brake fluid reseviour). Check the vacuum lines for leaks. If not, most times the master cylinder has been bad and leaking fluid into the booster which will damage it.

Hi:
The brake master cylinder is mounted to the back wall of the engine compartment, on the drivers side. It is almost directly in front of your brake pedal!

in the trans is 80w-90. the clutch fluid is actally the exact same as dot3 brake fluid.

Brake system problem or master cylinder low of brake fluid.

load sensing valve link between rear axle and body is out of adjustment. best way to fix is adjust in direction of downward body movement causes valve to open more. a 12mm spanner is all you need.