Dually Brakes

[Mild Thunder]

Ok guys. I have a 1989 2wd Dually crew cab. I bought the truck a couple months ago. Since I bought it, the brakes have sucked. Its like i have no rear brakes, or very little. Last weekend i had to make a hard stop at a red light, and the front wheels locked up, while the rears didnt.

So far i have replaced the master cylinder, rear wheel cylinders, new rear shoes and hardware, and eliminated the height sensing proportioning valve in the rear, as some suggested. Bled rear brakes several times. Shoes are adjusted so there is a little drag on the wheels while turning them by hand.

I thought of something today. It looks as if someone replaced the hydroboost unit, or fooled with it. Theres a crap load of RTV silicone where it meets the firewall. I can grab the master cylinder, and get some movement out of the whole master cylinder/hydroboost unit. How does this attach to the firewall? Im thinking maybe the rod that depresses the master cylinder or something along those lines is out of whack? Can this cause my problem? The pedal seems to travel quite a bit before i get any braking action. Main reason for going to a dually was big brakes! I wana get this figured out asap! Thanks!

[frito-bandito]

It bolts to the firewall with 4 bolts, a common problem is that the mounting area on the firewall has a tendency of wearing out and cracking, which causes the firewall to give when you hit the brakes resulting in less pedal travel which could very well be your problem. I've seen people weld the firewall or add metal and the weld the firewall, if they used RTV silicone for that reason, FIX IT IMMEDIATELY, it is a serious safety hazard. Keep us updated and good luck!!!!

[Mild Thunder]

took a closer look last night. The bracket that attaches to the firewall is good and tight. But the movement is between the hydroboost, and the bracket. What holds the unit to the bracket?

[frito-bandito]

Maybe check the mounting bolts on the hydroboost, if they're not under the hood, check under the dash where the brake pedal rod goes through the firewall. Make sure they are tight and that should fix the problem.

[Lt.Del]

http://forum.73-87chevytrucks.com/smforum/index.php?topic=5202.0

[jaredts]

That's good reading, SgtDel.  He has hydroboost, does the test mentioned apply for testing that (pump the pedal, start the engine and it drops)?  It does sound like a similar problem, just a different kind of booster.

[Mild Thunder]

Thought of something else this morning. Just for the heck of it, im gonna replace the rear rubber hose that goes over the axle. I wonder if its collapsed a bit, not allowing full pressure to the rear brakes. For 15 bucks its worth a shot i guess. Might as well do the front hoses too. Its a really clean rust free truck, that i believe sat for quite a while. Most of the stuff on it looks original.