diagnosing Brake Master Cylinder

[soundstudio315]

Anyone have the DIY version of this, I think my brake master cylinder is out, I have a lot of pedal travel:

INSPECT BRAKE MASTER CYLINDER OPERATION
(a) Inspect the battery voltage.
Battery voltage:
11 to 14 V
(b) Turn the ignition switch off and depress the brake
pedal more than 20 times.
HINT:
When pressure in the power supply system is
released, reaction force decreases and stroke
becomes longer.
(c) Check that the brake pedal reaction force
decreases.
If the pedal reaction force does not decreases,
check and replace the brake line and brake master
cylinder.
(d) Turn the ignition switch to the ON position and
check the pump motor operation noise.
If the pump motor does not operate, check and
replace the wire harness and pump motor (See
page BR-25).
(e) Connect the intelligent tester to the DLC3.
(f) Turn the ignition switch to the ON position.
(g) Select "ACTIVE TEST" mode on the intelligent
tester.
HINT:
Please refer to the intelligent tester operator's
manual for further details.
(h) Jack up and support the vehicle.
(i) Release the parking brake lever.
(j) Shift the transfer shift lever to the "N" position and
check that the rear wheels rotate by hand.
(k) Inspect front VSC solenoid (SMCF) operation.
(1) Select "SRCF (SMCF)" on the intelligent tester.
(2) Turn the "SRCF (SMCF)" ON with the
intelligent tester, depress the brake pedal with
stable force and check that the pedal cannot be
depressed.
If the pedal can be depressed, replace the
brake master cylinder.
NOTICE:
When operating it continuously, set the
interval of more than 20 seconds.
HINT:
To protect the solenoids, the intelligent tester
turns OFF automatically 2 sec. after every
solenoid has been turned ON.
(3) Release the brake pedal.
(4) When the solenoid is OFF, depress the brake
pedal again and check that the brake pedal can
be depressed.
If the pedal cannot be depressed, replace the
brake master cylinder.

 

[Kmvreter]

What year is the truck?

If it's a 4th Gen, then pop the hood open and roll down the driver's window so you can hear under the hood. Turn the ignition on but don't start/run the engine. Now pump the brakes many times to hear if the brake booster pump is cycling on and off. Let us know.

 

[soundstudio315]

What year is the truck?

If it's a 4th Gen, then pop the hood open and roll down the driver's window so you can hear under the hood. Turn the ignition on but don't start/run the engine. Now pump the brakes many times to hear if the brake booster pump is cycling on and off. Let us know.

Turns out it was a TSB brake pad shim that wasn't inplace. HUGE difference!