I really hate to start another thread on brakes, but I haven't been able to quite find the answer to my problem by searching. I have a '93 w250 that has had disc brakes put on the rear. They are front discs from a '76 chevy k30. I replaced the master cylinder and booster when I bought the truck in April. I bled and bled and bled the brakes until I've nearly gone through a gallon. I set the master cylinder rod length to the same as the old one, but I'm now starting to question that. My problem is that the pedal is super spongy if there's even any feedback at all. Then about half way through the brake pedal travel I get all the brakes and have to be careful not to lock up the brakes. I had finally thought that maybe that is just how the brakes on these trucks are, but I'm now more worried about the brakes and want to get them working how brakes should. Maybe not get the same performance out of them as my parents '02, but at least get a better pedal that can more easily be modulated and instills more confidence. I bench bled the master cylinder with the help of my neighbor who has done many many braking systems. I then installed everything with rod lengthy matching old master cylinder. Made brakes better, but still not good enough. Bled breaks several times over several days and wasn't getting any more air out. Any help would be appreciated. The truck is currently two hours away at my grand parents as I left it there the other day to go back for it in the next week. Thanks again!

 

You say the rears are disk calipers from an old Chevy? What position did the bleeder ports wind up in? The bleeders have to be at the very highest point otherwise you'll always have a bubble of air in there. Also, is there a RWAL module in the rears? It would be on the left side of the frame under the bed and plumbed into the real brake line. It might be giving you problems if it'e there.

 

The bleeders are on top yes. The RWAL module is there. I have bled it and I feel it engaged when the rear wheels start locking first.

 

My rear conversion calipers are from 76-77 Cadillac El Dorados and have the parking brakes. Do yours have parking brakes? If so, the adjustment on those is very important...Mark

 

The calipers are front calipers so they do not have the parking brake unfortunately. The calipers you have will swap right in and I plan to do so. The swap was done by the PO so I didn’t have any say in it. I’ll just have to figure out how to do the parking brake since the parking brake is MIA. The only part left is the part that hooks to the pedal that goes back to that piece of c channel that holds the rear cables for either side.

 

The egr kit with parking brake comes with the Caddy calipers, but has a custom lever that is used with the stock P-brake cables. It sounds like you only need the 2 rear cables and the Caddy calipers. The hard part would be the lever. I can post a pic of it and maybe you can duplicate it or maybe EGR will sell you a couple. All that said, try removing the caliper,but not the brake line, put it in a position where the bleed valve is straight up and just gravity bleed it, without pumping the brakes. The instructions that come with the Caddy calipers (not from EGR) said that is the only way to get all of the air out. It could be the same with the ones on your rig...Mark

 

Going to sound stupid, but are you sure the rotor isn't running out or is cocked relative to the caliper?

In either case, when you take your foot off the pedal, the rotor will shove the pads apart and that space has to be made up every time you hit the pedal.

Just a thought, with the pedal relaxed, try and run a feeler gauge between the pads and the rotor on both sides, if the pads aren't resting flat, fix that first.

 

Not sure. I wouldn’t think so, but when I get a chance to drive the 2 hours to my grandparents to get it back from do stuff over there I’ll have to check it out and try gravity bleeding it too. Maybe if that doesn’t fix it I’ll have to try messing with rod length some more. Battling a injection pump full of solidified veggie oil fats too from the PO. Hoping it comes out so I don’t have to spend the money on a new IP. Don’t really have the $1000 right now. Sorry. Now I’m off topic!

 

Well. I’ve gotten the truck back as some here know and have been working the kinks out of it. I gravity bled the brakes last week and it didn’t improve the pedal any. The pedal is still soft the first 1 1/2 to 2 inches, but will hold the truck in gear even where it is squishy. If I press the brakes hard it will lock up all 4 with the abs kicking in in the back. When I gravity bled the brakes I only got air from the driver’s rear. I need to get the elderado calipers next so I can have a parking brake again. I’ll probably wait intil after I get gauges and save up some money again. Just thought I would report back on gravity bleeding. The pads also seem to be parallel to the caliper so I’m fairly certain that isn’t the issue. I guess it is good enough. Should stop the truck well enough if it can lock up all 4.

 

Did you get an oem master cylinder or one that was designed for all disc brakes?

 

 

When you say OEM, that means the replacement master cylinder you got is the 40/60 design like the one that came from the factory. 40% front reservoir went to the rear drums, while the rear 60% reservoir fed the fronts. You now must get a completely different MC, one designed for disc/disc systems. The reservoirs are both split even.

Your factory differential valving system that sits under the master cylinder needs to be changed out also, as that is designed for disc/drum. Plus, it’s old. Get one made for all discs.

 

Gm disc/disc master cylinder and Hydro boost.

 

Well... that is jus great! I wish I would have known that before I spent ~$300 on a new master cylinder and brake booster last April. It made the brakes better as the booster was shot and the master cylinder wouldn't hold pressure, but now I've got to add something else to the list I guess. I'll probably have to wait to save up until after I get gauges and the calipers with a parking brake. What specific vehicle will bolt right up? I don't want to do a bunch of modification and I'm still in high school so I'm kind of on a budget.

 

There are some after market, generic Mc's that fit our trucks. Summit has them, I think...Mark Edit: Here are a few to browse through:

https://www.summitracing.com/int/sea...illocation=int