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  GEMCO Type H & HM Installation, Dimensions
     and Service Manual:
  Type H Brake System
  Type HM Brake System
  General
  Brake Dimensions
  Air-Over-Hydraulic (Type A/H)
     Brake System
  Air-Over-Hydraulic Parking
     (Type A/HM) Brake System
  Type A/H and A/HM General
     Information
  General Troubleshooting
 
 
 
 
 
 
 
  NOTE: Installation and service
  information for control cylinder,
  fluid reservoir bleeder, and
  pushbutton illustrated under
  Type H brake systems also
  apply to Type HM systems.
 		     INSTALLATION
  6 x 3 and 8 x 3 Type HM
     Brakes
  10 x 4, 14 x 6 and 18 x 8
     Type HM Brake
  Type HM Parking Brake
     Control Unit (Incl. J45767
     switch & light)
  Wiring Diagram Type HM
     Parking Control Unit
  Piping Diagram One Brake,
     6 x 3 and 8 x 3 Type HM
  Piping Diagram Two Brakes,
     6 x 3 and 8 x 3 Type HM
  Piping Diagram One Brake,
     10 x 4, 14 x 6 and 18 x 8
     Type HM
     SERVICE
  6 x 3 and 8 x 3 Type HM
  10 x 4, 14 x 6 and 18 x 8
     Type HM Brake
  Servicing 10", 14" and 18"
     Type HM Park Release
     Cylinders
 
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 GEMCO Type H & HM Installation, Dimensions and
 Service Manual:
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SERVICE INSTRUCTIONS
10", 14" And 18" Type HM Park Release Cylinders
 
The HM parking cylinder may be overhauled using repair kit J73921. However, three special tools are needed to safely and correctly disassemble and reassemble this cylinder.
J391, Spring Clamp Tool. This tool is needed to clamp and hold the setting spring under high force while the piston rod nut, piston and other parts are removed from the piston rod. The tool consists of two matched pairs of clamps, held together with four Allen head cap screws and two through bolts and nuts that clamp and retain the setting spring under 300 pounds force.
J394, Expanding Drift Tool. This three-piece tool is used to remove and reinstall the O-ring retainer from the cylinder body.
J392, Pilot Tool.This tool is needed when reinstalling the piston rod back in the partially reassembled cylinder body.
DISASSEMBLY AND REASSEMBLY OF HM PARK RELEASE CYLINDER
Disassembly
  1. Clamp J 391, Spring Clamp Tool, on the brake setting spring. Tighten Allen head screws securely, then tighten nuts on through bolts to compress spring until it turns freely on cylinder. CAUTION - THE SPRING IS NOW UNDER 300 LBS. LOAD AND DANGEROUS. IT SHOULD REMAIN CLAMPED UNTIL IT IS REASSEMBLED ON CYLINDER ASSEMBLY.
  2. Lightly clamp cylinder body (1) in vise and remove cylinder end (16) with mounting lug (21).
  3. Now, clamp spring-retaining shoulder (not shaft) of piston rod, remove piston rod nut (15) and lock washer (14), and pull piston rod (8) from opposite end of cylinder. Remove spring and spring clamp from cylinder body.
  4. Remove piston (13), cup (12), cup retainer (11), piston stop sleeve (10), and gasket (9) from cylinder. If necessary, push parts out from opposite end of cylinder using 3/4" rod.
  1. Insert J 394 Expanding Drift Tool through cylinder until seated, then fully tighten the expanding drift and tap out the O-ring retainer (6) as illustrated above. Then remove the O-ring (5) and bushing (4) through cylinder.
  2. 6. Now remove four packing felts (3) and felt washer
Inspect the bore of the cylinder. If it is pitted or scratched, replace the entire HM park release cylinder. If the cup or O-ring is very soft or enlarged, this is evidence of brake fluid contamination and the entire brake system should be flushed and all other components rebuilt or replaced. Inspect J9674 bushing (2). If necessary to replace, rebore to .829"-.831" diameter.
 
Reassembly
 
Reassemble all parts in reverse order, using all new parts from repair kit J73921. Before reassembly, wash all parts clean with denatured isopropyl alcohol. Then lubricate all parts before reassembly using Heavy Duty Brake Fluid.
 
Reinstall bushing (4) with new O-ring (5). Now use J 394 expanding drift tool and tap new O-ring retainer (6) in place next to O-ring. Remove tool. Now dip new packing felts (3) and felt washer (7) in brake fluid and insert through small end of cylinder body. 		Reinstall brake setting spring with spring clamp tool on cylinder body. Do not remove spring clamp tool. Now slip J 392 pilot tool over threaded end of piston rod and insert rod through small end of cylinder body. Remove pilot tool.
 
Slip new gasket (9), piston stop sleeve (10), cup retainer (11) and cup (12) into cylinder and over the piston rod (8). Now, holding shoulder of piston rod against setting spring, slide piston (13) in place with square cut recess of piston toward cup. Reassemble lock washer (14) and nut (15). Tighten to 60 ft-lb torque.
 
Before replacing cylinder end (16), disassemble mounting lug (21). Dip new filter (20) in brake fluid and install with retainer (19). Reassemble mounting lug and cylinder end.
 
Remove spring clamp tool and reinstall cylinder on brake.
HM PARK RELEASE CYLINDER REPLACEMENT PARTS LIST
Item
No.
 Name of Part
10" x 4 "
14" x 6 "
18" x 8 "
  Complete Assmebly
J-79463
J-79462
J-79464
21
 Mounting Lug
J-9681
J-9681
J-9688
8
34
35
 Piston Rod
Piston Rod Guide
"C" Washer
J-23040
J-9662
J-9661
J-23041
Not Used
Not Used
J-23041
Not Used
Not Used
The following parts are used in all the above Releasing Cylinder Assemblies:
Item
No.
 Name of Part
Number Used
Part Number
1
2
3
4
 Cylinder Body
Bushing
9691 Packing Felts
Bushing
1
1
4
1
J-14704
J-9674
*
J-9672
5
6
7
9
 10167 "O"-Ring
10229 "O"-Ring
10231 Felt Washer
1733 Gasket
1
1
1
1
*
*
*
*
10
11
12
13
 Piston Stop Sleeve
9696 Cup Retainer
9694 Cup
Piston
1
1
1
1
J-9697
*
*
J-9695
14
15
16
 Shake-proof Lock Washer
14707 Piston Rod Nut
Cylinder End
1
1
1
*
*
J-79467
19
20
 9680 Filter Retainer
1823 Filter
1
1
*
*
31
32
33
 25 lot pack F17 Gasket
Bleeder Screw Adaptor
Bleeder Screw
1
1
1
J-105376
J-7346
F-6446
* ORDER J73921 HM CYLINDER REPAIR KIT