CMP brake question

[Phil Waterman]

CMP brake question about both Ford & Chevy C30, C60, and C-GT and their Ford equivalents.

The issue is the, minor or upper adjustment, interval how many miles between adjustment are people getting? My C60S Radio body truck with the 261 engine, curb weight 12,000LBS, gets between 1000 & 2000 miles a year. My routine has been to adjust or check the adjustment each spring when do the general servicing. With the truck up on jack stands it is just makes the whole servicing easier.


Now normally over the years it has taken only the most minor adjustment of cams A & B (Fig. 6 - Adjusting Cams) maybe a few degrees. Once this is done the brake peddle is right back up at the top of the stroke.


Figure 6 Adjustming Cams.png


The last three times I have done this three wheels RF, LF, LR all needed almost any nothing, but the Right Rear both A & B have needed much more. What I also noticed was that they turned much easier than all the others.

Has anybody else noticed a similar problem?

I had adjusted the brakes 3 months ago, recently I was not happy with the brake performance, peddle goes down about halfway and with a quick pump brings the peddle right back to the normal top point. So put the truck on jack stands and checked the adjustment and again the Right Rear was out of adjustment again.

Cheers Phil

[David Dunlop]

Phil.

Is there anything significantly different about the brake assembly components on that wheel from the other three, even if just different in maker?

Can the adjusting cam assemblies themselves wear out and require replacing for any reason? Seems a bit odd that set are easier to deal with than the other three.

David

[rob love]

The same bendix system was used on the M35 2-1/2 ton trucks, the 5 ton trucks, the 3/4 ton Dodge, right down to the M38 Jeep. I always did a major adjustment when the brake job was first done, followed by a couple of minor adjustments. At the end of the day, you need the contact to be in the center of the shoe's arc. Repeated minor adjustments won't achieve that once appreciable wear has occured. I never had to do them as often as you are doing. Some variables are the condition of the drums (smooth/scored), the composition of the linings, the type of driving, and possibly the over adjustment of the shoes in the first place.



I would suggest you try a major adjustment again. Turn the lower anchors in the same "waterfall" type direction as the upper adjusters. Some of the bendix drums will let you get a feeler gauge into the drum so you can properly set them up. What you are after is contact in the center of the shoes.....not top or bottom.



If the shoes are correct, and you are still findng the need to pump the pedal, then perhaps a good bleeding is in order. Fresh fluid in the system is a good thing, and a preventative maintenance item that is often overlooked.

[Phil Waterman]

Hi All

First thanks to Rob and David for responding.

First to David's comments/questions:
I don't think there is any significant differences in the pairs of backing plates. But David your comment made me check the position of all eight of the lower/major adjusters. The 4 rear ones were dots pointing toward each other. While the 4 front ones are at the 2 and 10 o:clock positions.
C60S Major Brake Adj LF_2426.jpgC60S Major Brake Adj LR_2422.jpg

Second Rob's comments- "I would suggest you try a major adjustment again" Well I did a careful major adjustment per the manual:

Manual 2.jpg

10. Was getting a stead 30 lbs. pressure on the peddle.

Getting just 30lbs peddle pressure is a touchy thing, wish the manual explained the logic. You can see from the photo how this was done with a 30lbs with hanging pulley so as to put just that pressure on the peddle. This pressure seems to just take up the play in the wheel cylinders, without applying the brakes to drag. If you push down on the peddle just a little more and release the brakes apply and the wheels don't turn. So I went with just 30 lbs pressure.
C60S Major Brake Adj_2418.jpgC60S Major Brake Adj_2420.jpg


11. With a short (3")wrench turned the two lower pins on each wheel just the smallest amount to just cause brake drag/sound. Repeated this on all the wheels carefully re-tightening the locking nuts. When the pressure was removed the wheels turned smoothly with no drag.



Robs last point is well taken "If the shoes are correct, and you are still finding the need to pump the pedal, then perhaps a good bleeding is in order. Fresh fluid in the system is a good thing, and a preventative maintenance item that is often overlooked." Because all CMPs have single brake systems I take preventative maintenance seriously. Any sign of leakage means full brake job. No leakage every 9-10 years replace all the rubber parts in the brake system, cups, boots, lines and of course flush the brake system. Last done on this truck 2015.


Ok after very carefully doing the FULL brake adjustment procedures tool the truck out for test drive, following observations:


1. Peddle right at the top of the stroke
2. Brakes apply very briskly
3. Did several full hard stops from 30-40 MPH nice and straight with good braking. Could not lock the wheels but, but close. Now stops nicely on 9% down grade, which was one of my prior concerns, it just didn't feel like it wanted to, now it stops nicely.
4. After 6-10 hard stops checked the brake drum temperatures LF 136, RF 122, LR 130, RR 129 Degrees Fahrenheit with IR Gun.


Only time will tell if the Right Rear goes out of adjustment, but I have added witness lines to all of upper adjustment so will be able to tell if its shoe wear or the adjuster moving.


Cheers Phil

[rob love]

The 10 and 2 sound more correct. Both dots facing each other would be maximum retraction of the shoes. The shoes are likely wearing more at the top in that case.



Using the weight is interesting. Never seen that done before.



One more thing to consider is if there is a long lining shoe and a short lining shoe, and that they are in the correct positions according to the manual. That could lead to excessive wear on a shoe if it is in the wrong spot.

[Tony Smith]

Regarding the "Major" adjusting bolts, the manual says there is a special tool (a long oval-shaped hole in a flat bar) that fits the head of those bolts. It wasn't a military tool, it was a regular truck part, but i have never encountered one for sale. I have just used an adjustable spanner on the flat sides, but I don't think that is the best option.

Do any modern US tool makers like Snap-on, Proto or others make the flat-bar tool or a special shaped ring spanner for these bolts, seeing as it is a regular truck adjustment?

[rob love]

I have never found a modern version of the tool, but have picked up originals at various military shows as well as various military surplus dealers.