By John Gibbins M.I.A.M.E.
Some of this information is compatible with many US and Canadian manufactured WWII Vehicle basic braking systems up to and including some 6x6 trucks.The jeep braking system is fairly simple compared with modern variants but there are some idiosyncrasies and basic stuff one should/must know about.This is not rocket science and one shouldn’t need specialists to help overhaul or work on their braking systems.
Fluid is held in a contained system and a force applied by the push rod to the master cylinder is transferred to other (restrained) points in the system (wheel cylinders and shoes).
MASTER CYLINDER & COMPONENTS
1. The Cast Body with vented filler cap and copper/aluminium gasket seal (bore can be reclaimed with stainless steel or brass sleeve and sometimes honing)
2. Aluminium piston with holes and a thin copper plate flush riveted to the main face( mates up with the primary cup or seal)
3. The piston is retained in a returned position with a spacer and a circlip. If the spacer is left out this will show up as too much pedal travel before braking commences when other adjustments all appear correct.
4. Rear or Secondary Cup (will leak around the dust boot if faulty. Provided there is sufficient fluid in the reservoir will not cause brake failure or loss of braking)
5. Primary Cup or Seal has a brass shim between it and the piston copper plate (shim must be in place. Also, total loss of brakes or pedal slowly sinking to the floor are symptoms of a failed or failing primary cup)
6. Two holes in the reservoir through to the cylinder.
7. The Charge or Fill Port 4mm (3/16”)
8. The Compensating or Equalizing Port with small hole <1mm (<1/32”) This hole must be clear when the piston is fully returned otherwise brakes will drag and excessive heat will be exhibited at the drums, up to and including brakes locking up.
9. Piston Return Spring with a residual pressure check/pressure valve fitted in the brake line end of the spring. A flat rubber seal the check valve seats on at the end of the bore.
10. A hydraulically actuated pressure switch completes a circuit for brake lights. To check for brake light operational problems join wires together and lights should work, if so, switch is the problem and not wiring or bulbs.
11. Copper washers either end of brass brake fitting. Should be replaced or annealed (heat and quench in water) if Old and/or Hard.
How It All Works
§ On application the piston moves forward a very small amount and the primary seal closes the compensating/equalizing port creating a fully sealed system.
§ There can be a small surge of fluid in the master cylinder before the seal covers the port.
§ As fluid is incompressible (as far as we are concerned) it moves as a solid medium opening the little check valve and pushing out the sealing cups and pistons in the wheel cylinders applying the shoes to the brake drums.
§ At this point (when the shoes contact the drums) pressure starts to build.
§ If the pedal feels hard, that is good.
§ If it is spongy there is the possibility of air in the system.
§ When the pedal is released the brake shoe return spring retracts the shoes (unless you have binding or seized w/c/s) forcing back the m/c piston and diminishing the pressure in the system.
§ This creates a lower than atmospheric pressure in the previously sealed system as the fluid pressurized in the system can’t get back into the reservoir quickly enough (small equalising port won’t let this happen and don’t go drilling this out)
§ As the piston moves back quickly the charging/fill port, at atmospheric pressure allows adequate supply of fluid which flows through those little holes in the piston collapsing the primary cup and filling the “void”.
§ This eliminates any problems with the need for repeated quick applications. It also explains why a HARD pedal comes up higher when brake adjustment is needed.
We add an extra amount of fluid in the lines before everything has fully returned to its rest position and in effect the brakes are dragging.
You will now understand why the copper valve plate and brass shim is in front of the piston. If they weren’t the rubber would be eroded away like a bottle top (cap) after extended service causing non sealing and “brake failure” on application.
Signs would be pedal sinking to the floor on application and fluid return through both ports exhibited in the m/c reservoir while applying.
This does not feel the same as air in the system and there could be no visible fluid loss.
§ The piston and primary seal when fully retracted uncover the compensating/equalizing port, atmospheric pressure is on all fluid and any extra fluid returns to the reservoir through the small compensating port hole and the system is ready to do it all again.
The little check valve in front of the piston return spring (shown in the diagram) reseats and maintains a line pressure of about 25kPa (3-4psi) above atmospheric, not enough to cause brakes not to return but enough to cause the w/c seals to be kept nice and tightly in place ready to go to work again.
Master (and Wheel) Cylinder Servicing
§ Repair kits are available and sometimes the m/c and w/c’s can be reclaimed by re-sleeving or honing and fitting these kits .
§ Oversize cups are not available, as they once used to be.
§ Honing is not meant to remove much material at all just clean up the bore which I would use fine steel wool on first .
§ If there is any pitting or indentations remaining in the bore caused by corrosion (or whatever) the cylinder is not re-useable without re-sleeving.
§Fit up overhauled cylinders with special purpose rubber grease (no mineral product allowed).
§ Keep in mind the weakest link will fail and when that is repaired the next weakest will fail.
§ RECOMMENDATION is all or nothing.
§ Brakes are one area to do best the first time and save bucks down the line.
§ Shop labour costs in repairing will be higher than buying all new stuff and doing it yourself.
§ Sleeving is a great (one time expensive) option to keep original and get long, long life.
Steel Brake Lines
§ These are made up of special steel tube made for brake lines, double flared at all extremities. They should not be made from copper or single flared. In some cases stainless steel lines are used when salted roads and corrosion are issues.
Double Flared Steel Brake Tube
§ With the right flaring tool you can do it yourself with some practice .
§ Some little “ S ” bends are a little harder than straight lines as we know!
§ Bending should be done with special benders and in some cases inside coiled springs meant for that purpose .
§ Without you will kink and ruin the tube, so don’t try!
§ All lines should be fixed with original type clamping (or similar) in all original positions. Fatigue and failure if not done properly .
Flexible Brake Lines
Short and long shoe linings
Backing Plate, Anchors, Adjusters and Wheel Cylinders
Anchor eccentric adjusters
§ Lightly loosen the forward located (leading) brake shoe eccentric lock nut with a ring spanner (box wrench) and leave it in place so that you can retighten in one (1) go. We are talking of the top adjuster!
§ There is a special tool (Snap On) 3/16” or equivalent metric OE wrench (my kit does not go down that far) and at worse a 100mm (4”) shifter or adjustable and no bigger will do the job.
§ “If” the top adjuster has a dimple and it is away from the wheel, the cam is off
§ The adjuster should not feel real tight, if it does loosen the lock nut further (slightly) and do not get a bigger (more leverage) type tool 3-4” is enough.
§ The cam is symmetrical so you can turn either way but it is best to get used to a certain method e.g. out toward the wheel for the front (leading) and back toward the wheel at the rear (trailing) then when you adjust next time you turn the same way and get immediate results.
§ This adjustment realigns shoes with the drums after re-line, drum skimming, new drums and the like.
§ OEM drums are slotted to fit feeler gauges to measure clearances; some aftermarket drums do not have these slots.
§ One could drill a small hole (1/16”) depth limited in each drum so that wire gauges could do the job, being extremely careful not to drill into the finish drum to shoe bore.
§ If the drums are worn and linings are scored go straight to the section NO SLOT METHOD
§ You can do your minor adjustment as related above and the top major reading .008” should be pretty right but recheck anyway.
§ Major Adjustment is accomplished by turning the anchor pin dots toward each other and then down until the shoes are set to the proper clearance using a feeler gauge
§ The recommended shoe setting is .008” at the toe (upper end) and .005 clearance at the heel (lower end) each 25mm (1”) in from the lining extremity
§ Loosen the anchor lock nut on the forward primary shoe (not too much) leaving the wrench attached (notice how Derek has a permanently modified and shaped a wrench to fit above).
§ With the feeler slot or drill hole 25mm (1”) in from the end of the toe (top) of the leading shoe check and adjust for .008” by turning the anchor dot downward
§ Move the drum so as to have the slot 25mm (1”) from the bottom (heel) of the same shoe, check and adjust to .005.
§ Tighten the locknut not allowing the anchor bolt to move while doing so otherwise the adjustment will alter
§ Check for drag and if dragging do again
§ Mark locknut with chalk when done
§ Then do each other shoe on the vehicle the same way remembering that the trailing shoes are shorter so the position of the feeler will be different
NO SLOT METHOD
§ I will offer another method not requiring the slots or drill holes lifted from a WWII publication related to light Chevy Blitz trucks (thanks to Jimmy Sewell, an old buddy from TAFE days) which is simple, quick and easy. Presumably it will work on Ford GPW’s with no ill effects from cross pollination, envisaged!
All the IMPORTANT things done?
Do your minor adjustment as related above
§ Apply a force of 25-35# to the brake pedal. This force must be consistent and maintained through out the entire adjustment.
§ This is not much force; place a small piece of wood in between the pedal shaft in the engine compartment and the firewall holding the pedal down slightly and consistently just past free play.
§ This force moves the m/c piston enough to close the compensating or equalising port but not move the wheels cylinder pistons!
§ All bottom anchor pin dimples should be as close as they can be together (adjacent to one another)
§ When their locknuts are loosened we use the system of leaving the ring spanner (box wrench) on the anchor lock nut as we did above
§ Adjust each of the 8 anchor pins (in turn) moving the dimple downward with no more than 3” long wrench until the shoe just touches, indicated by very slight interference when turning the drum, back off a fraction
§ Tighten the anchor pin lock nut as you complete each one ensuring the anchor pin does not move and mark as done with chalk
§ Remove the pressure from the brake pedal and make sure all the brake drums turn with absolute minimal to no dragging
§ If the shoes drag you need to do it again. Be more careful this time as 4 stubbies (beers) affects concentration!
§ Road test when under .05 in an area with little traffic. After a little bedding in try a few repeated hard applications.
§ Pulling up in a straight line ?? you’re a winner!
Sketches and diagrams are courtesy of the odd g503.com post, Bendix, Snap On and TM’s.
Jeep MB/GPW Foot Brake Major adjustments made simple [diagram 1]:
Brake shoes must be concentric with drums. This can’t happen with skimmed drums & standard shoes
[see diagram 2]
1. Turn all cam adjusters to fully retract all shoes
2. Turn all bottom anchor adjusters so dots [or arrows] face one another
3. Turn anchor in direction of arrow/s until the shoe heel/s drags on drum
4. Turn top cam/s in the arrow direction of the arrow/s to bring the shoe into contact with the drum.
This will cause the shoe/s heel/s to lose contact with the drum
5. Turn the anchors again in the direction of the arrows until the shoe heel/s just drag on the drum/s again
6. Repeat step #4
7. Repeat step #3 & #4 until cam adjustment no longer frees the shoe/s heel. Then tighten the anchor bolt nut/s
8. Repeat procedure for all shoes if you have not already done so
1. Check the operation of the the handbrake lever and pawl locking mechanism. Note:- When applying the handbrake always turn the handle 1/4 turn (the same you do when releasing) then turn back to lock when application is complete. This will save Pawl wear & tear.
2. Establish that all cable attachments are in place, tight, and that the outer cable is firmly secured at the fire wall and other extremity.
3. Lubricate between the inner and outer cables, ensuring free easy movement. ( This lube should be done as an ongoing service basis)
4. Disconnect the cable at the Drum end. This ensures the cams(4 of) etc. are all in the off position.
5. Check the brake band is in place correctly, correct springs and all hardware attached.
6. Elongated or oval holes and worn clevis pins (4 & 5) and such may need components and holes built up, re-drilled and pins replaced.
7. Anchor (1) is your first point to adjust and there are two things of importance. Clearance between the transmission lug to band attachment should be no more than 0.005". If larger, remove the band and adjust the gap by squeezing in a vice, gentle persuasion or other suitable method. When fitted there is between 0.005" and 0.010" drum to lining at that anchor. Lock wire must not be overly tightened or binding.
8. Adjust Nut (2) until the Lining just binds with the Drum.
9. Adjust Screw (3) until the head engages on the Band and the Nut and Locknut are just snug against the Bracket (6) Clearance should be around 0.010" when this happens.
10. Back off Number 2 Nut, two full turns, or give the Lining approximately 0.010" and the Drum should turn freely.
11. Re-attach the cable, and if it doesn't match up properly, the outer cable positioning needs altering so everything lines up.
12. The original MB/GPW cable was not adjustable, but by now, modifications done so that it can be.
13. This brake is meant for parking and should not be applied when the vehicle is in motion unless in an emergency situation.
14. The faster you go the harder they wrap on.
15. Regardless of what some may think, they are very efficient and serious damage may occur, I have struck a tail shaft that looks like a corkscrew, so be aware.
If you don't have any hassles, it should take less than 20 minutes to obtain the best results.