I am posting the gory details of my rebuild of the two Midland 490-g brake boosters on my 69 Glastron which lives on a M-300 Dodge chassis. What led me to do this job of course was necessity. The symptoms of the booster problem was an unyielding brake pedal even with the engine on. I had brakes but not power brakes. I have a pretty good M-300 Dodge maintenance manual but it only covers 1970 and 71. Fortunately for me the boosters shown in the manual are very similar to the one on my chassis. I did find out that my chassis is actually a 1968! The point here is to make sure you have a good exploded view of the booster you are going to work on or trouble will surely ensue!
Now a small disclaimer. I had never done this particular job before so I did get some information from an online source and of course parts which I will post links to these sites below. I am an experienced mechanic and have worked on everything from Jet fighter aircraft to 747s, race cars, race motorcyles, etc. If it makes heat smoke and noise I have probably worked on it. With that in mind i would rate this job on a scale of 1-5 at a 3.5. I do not mean to imply that you must have the background that I have to do this job but it would help if you have a good selection of tools, a good digital camera and maybe and experienced neighbor or friend who can help out. I am 70 years old so this will be a piece of cake for most of you young bucks!
The first thing is to get under the rig and locate the boosters. My chassis has two separate units, one for the front brakes and one for the rear. They are totally separate from each other. The system also has a Master cylinder up by the brake pedal which supplies all the fluid and pressure to the slave boosters. I dropped the intermediate driveshaft from mine to get some more room. Disconnect everything from the boosters including, two brake lines, two vacuum lines,brake light wiring, and the line for the intake/exhaust filter. Mine was held in place by two bolts on a bracket each. The units weigh about 15 lbs. so don't drop them on your face!
After you get them out carefully take them apart and take pictures of what goes where and which direction the cup seals face either front or rear. After dis-assembly I used a wire wheel to clean all the gunk off the exterior of the units and small parts. Don't lose any of the little clips as they are not included in the rebuild kits, at least not the ones I got. Do not damage the surface of the shafts from the large bellows, they must have a mirror finish and if they are rusted or damaged will need to be replaced. Do not clean any of the parts with anything but Isopropyl Alcohol. Just go to the closet where your wife keeps this stuff and commandeer the quart bottle. If she resists, you are gonna need to go to the store! Getting the shaft nut loose from the shaft on the large bellows is a pain! I used two blocks of wood, one on each side of the shaft in a 4" vice and pulled it tight. I also used the slot on the end of the shaft for a screwdriver with a vise grip pliers and along handled 9/16 box end wrench and it finally came loose!
Inside the slave cylinder is the piston, cup seal, spring and end cap, also in there is a poppet valve that you will need to dis-assemble, clean well and then reassemble. On the control valve which is attached to the side of the slave unit, you will remove 4 each bolts and take the top off. The manual says to put a mark on each side of this unit so it can go back in the same place but I found that not really necessary as it has a vacuum line on it that goes to the end of the booster chamber and this can only go on one way.
Don't forget to look in the top of the control valve as it has a check valve in there and you will need to replace the o-ring. Just make sure you keep everything very clean and don't use anything but alcohol for cleanup and you will be fine. Use brake fluid for an assembly lube on the piston, cup seals etc. Re-assemble the whole works, double check all work as you go. Remember, if you put the cup seals in backwards the unit will not work! Now put it all back together the way it came apart and give her a coat of primer and paint and then back into the rig. Don,t panic if you are fearful of putting it back wrong, you can always do it over? No, seriously, if you do not get it right you will still have brakes as the master cylinder will pass brake fluid through the slave cylinders and work the brakes. You just won't have POWER brakes. If you are concerned about not having the skills or the tools to do the job you can send them to Precision Rebuilders INC
350 N. Commercial AVE. St. Clair, MO 63077 Call them at 636-629-1444. Ask for Amy, she knows her stuff and can get you kits or arrange for you to send your units for them to rebuild. They quoted me $180.00 each to rebuild. I got kits for $80.00 each. Doing it myself saved me $200.00! You can also get some good info from davesmopar.com