Not sure if this will help. I dont have access to Mitsi manuals but I think the TCM FD70 is pretty much the same and this is all it says about hard to stop. it also brakes down the operation of what they are calling a hydromaster which im guessing is the booster.

Trouble Shooting

Pedal travel is normal, but braking effect is poor

Inside of power cylinder is poorly lubricated or air leaks from power piston or power cylinder due to damaged packing = Replace packing or lubricate power cylinder.

Air leaks between power piston shaft and piston = Retighten

Poor contact of vacuum valve = replace vacuum valve and atmospheric valve as a set and calk them to 17mm

Hydraulic air cleaner is clogged = disassembly and clean.


5·2·5. TEST METHOD OF "HYDROMASTER"
(1) Performance Test
Remove the plug at the back of the "Hydromaster" and install a vacuum gauge. Start the engine and idle at approximately 450 rpm. Keep the engine in this condition for more than 130 seconds with the revolutionary speed of the
vacuum pump at 700 rpm. The vacuum gauge should read more than 600 mmHg.
Next press the accelerator pedal to increase engine rpm to about 1800 rpm.
The revolutionary speed of the vacuum pump is approximately 2500 rpm. Keep
this condition for about 80 seconds. The vacuum gauge should read 700 mmHg
or higher.

(2) Vacuum Tighteness Test
Gain a negative pressure of 500 mmHg by the same procedure as for the performance test and shut down the engine. lt is normal if the gauge shows a decrease of less than 25 mmHg per every 15 seconds. If a decrease of more than the specified is observed on the gauge it is an indication that the check valve does not have sufficient vacuum tighteness or there occurs vacuum leakage from the joint of the negative pressure pipe. (As a substitute procedure for vacuum tighteness test, remove the plug,send a compressed air of 1 kg/cm2 into the "Hydromaster" and immerse it in water. )

(3) Capacity Test of Power Cylinder

(a) Produce negative pressure of about 600 mmHg inside the power cylinder.
(b) Shut down the engine. Fully press the brake pedal and release it at once.
When the pedal is pressed, air suction sound is produced by the "Hydro master" air cleaner. The power cylinder is considered normal if this sound is
heard once out of every 3 to 4 times the brake pedal is pressed.

If I find anything else ill let you know. the manual has diagrams I cant post so idk how much this will help if at all.