Posted: Tue Apr 27, 2021 7:54 am Post subject: M38A1 brake shoe shimming required after drum turning?
Had a brake drum turned that was out of round.
This M38A1 has had a soft pedal since I got it.
PO put in new lines, MC, shoes, wheel cylinders and system has been bled. Brakes no worse since drum turned.
Adjusted eccentrics per manual.
I know the Army manual does not address this, should brake shoes on the A1 be shimmed after turning a drum?
If so, where do the shims go to make up for the loss of material?
I see the civy manual mentions this in Wes photo album, but photos end before stating the location.
After turning drums the inside diameter has increased. Therefore the shoe material was be relocated closer to the drum's friction face. Therefore the shims go between the brake shoe pad and the brake shoe. This means re-riveting the shoe pads on.
A soft pedal or a spongy pedal. Soft pedal is usually the result of a fluid leak out of the system or a fluid bypass issue with the master cylinder's piston. Drum diameter effects pedal travel but does not cause a soft pedal. _________________ Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100
Pedal goes about 1/2 way to floor, with correct 1/2 free play.
Pedal firmness does not does improve by pumping - as it does with air in system.
You have to push hard - it just does not feel solid and firm.
A puzzle since everything is new.
Now... the PO installed DOT 5 silicone fluid.
My first experience with silicone brake fluid and I know Wes does not like this fluid.
Could that give this "soft" brake pedal and the need to push harder than I am used to with old Jeeps?
Soft pedal was and always will be a problem with silicon brake fluid DOT5. That's one of the reasons the auto manufacturers in the 60's/70's dropped it.
Remember, now you cannot just switch to DOT3 or DOT 4 without first thoroughly flushing & rebuilding or replacing all components and replacing all rubber parts! _________________ Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100
To return to good old DOT3, would a complete flush with alcohol do it?
Or will the master cylinder and all wheel cylinders need to be replaced? Sounds like my long/spongy pedal problem has nothing to do with turning the out of round brake drum - the long/spongy pedal was already present.
I've learned the compressibility of the silicone-based fluid is up to three times that of the more common glycol-based types, so needs more pedal travel to actuate the brakes.
Not sure I like the PO's decision to replace all parts, then fill with DOT 5!
Or will the master cylinder and all wheel cylinders need to be replaced?
You just need to do as I posted above. You will have to:
1 - thoroughly flush the entire system.
2 - Replace all rubber parts.
3 - Rebuild/replace any parts found to be worn excessively or damaged. _________________ Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100
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