Clark GPX30E:
inch/brake master cylinder bore sizes?

anyone know master bore sizes? redid everything except brake master, got good pedal height, good park brake, heavy pedal though... suspect our ex 'mechanic' may have put two inching, or a otherwise large bore brake master on it...dont feel like tearing apart again right now, hoping for some insight on what bores are/if different to see if possible its got two inching, or just wrong brake master... everything else is new/bled/adjusted
  • Posted 9 Oct 2018 11:05
  • Discussion started by tc429
  • Ohio, United States
Showing items 1 - 3 of 3 results.
yes, extra pedal effort needed- it works but takes too much compared to others ive driven.

no booster on these trucks, all manual, put new wheel cylinders and new shoes a few days ago, old ones appeared oil glazed(seemed likely axle seals, but they looked dry- maybe they just didnt clean the stuff or spilled a little axle oil), both wheel cylinders were starting to seep into the boots, one boot had just started seeping into the brakes(thing sat abandoned outside 2 yrs, surprised wasnt even worse)...anyways all new, cleaned,adjusted, bled, bleeding the inch cyl was a hassle but brakes were easy to bleed.

everythings good now, just takes more pedal effort than i think it should...as the only remaining old part is the brake master, went ahead and ordered a 3/4"(still betting the wrong one was put on long ago)

other projects to get done around the house before winter, doubt i'll get back after it for a few weeks- its fine till whenever...agree this is one not so great design
  • Posted 12 Oct 2018 09:51
  • Reply by tc429
  • Ohio, United States
You say you have a heavy pedal? I'm assuming you mean your stopping power is not sufficent and have to press extra hard on the pedal to get the truck to stop?

before you go putting the wrong master cylinder on the machine have you checked the following?
- check for external leaks in the braking circuit (wheel cylinders included)
- check the booster assy for internal leaking (if equipt with a booster)
- Pedal freeplay is set correctly
- sticking components or fluid blockage in brake circuit (lines or junction blocks, etc)
- is the correct fluid in the system?

These machines have the vertical cylinders mounted underneath the dash between the inching and brake pedals if i'm not mistaken and according to the diagrams i'm looking at.
If this is true to your machine i can say they can be very difficult to bleed out and setup. IMO one of the worst designs clark came out with ever!
All i can say is good luck sorting this out, it will take some time and you'll probably need a pressure bleeder to bleed the system correctly.
Make certain you do not have fluid leaking into the brake chambers in the wheels, wet shoes is a common problem that cause poor braking issues.
  • Posted 10 Oct 2018 22:45
  • Modified 10 Oct 2018 22:50 by poster
  • Reply by swoop223
  • North Carolina, United States
You've been swooped!
swoop223@gmail.com
hard to find info... but, found(according to the internet...) that:
part number 2779923 = 15/16 bore (~.69 sq.in. piston area)
part number 2812896 = 3/4 bore (~.44 sq.in. piston area)

both numbers were found as replacements for a GPX truck

so *if* its got the bigger one, changing to the 3/4 will give ~56% increased PSI at same pedal effort...sounds likely the issue think i'll order one :)
  • Posted 10 Oct 2018 09:01
  • Reply by tc429
  • Ohio, United States

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