Bad master or booster
We bled brakes on my brothers 92 W250 Cummins Ram and he drove it and was able to lock up brakes, it stopped fairly well. After a bit he drove it again and I drove it as well. If you have to stop suddenly brake pedal goes 3/4 to the floor before it starts applying brakes, almost like theres air in the system. There was no air when we bled it. Also when pumping brake pedal with engine off, pedal does not pump up, almost like there's too much freeplay. Does this indicate bad master cylinder?
I am assuming Hydro-boost? Yes it sounds like Master has gone out.
Any other issues with the rest of the system? Calipers, Pads, rotors, shoes, drums, wheel cylinders?
this truck doesnt have hydroboost, its still old style Booster/master cylinder. Booster gets vacuum from factory installed vacuum pump.
Yours still sounds like master going bad, and depending on mileage changing the master doesn't hurt anything. Pain in the rear and wallet maybe, but it is 27 yrs old and if it's original OEM, replacement even if found to not be the problem just ends up be predictive/preventative maintenance.
With current quality issues I have experience the past few years, i will not do a "rebuilt" Master cylinder, too many field failures. I only buy new, name brand if I can.
On the comment of locking the brakes, if the rears locked and front didn't, it could be an issue with the combination valve, but with the pedal going down that far before it starts braking, I would keep with the simple stuff first and still inclined to think its the master.
If yours has ABS It would need to be properly checked. Most systems back then were fairly simple, but ABS was still fairly new. You will need a tech or shop that knows that system.
Historical from factory known issues: there were 4 known major issues and one of these issues on the 92 W250/D250 was with the rear shoes on the 12 inch drums.
Problems/Dodge/Dodge W250/Revised rear brake shoes
Vehicles equipped with 12" diameter rear brakes may encounter premature rear brake shoe wear.
Revised brake shoes are available for this concern.
1990, 1991, 1992, 1993
Does indeed seem like a defective brake master cylinder.
When you installed the new pads in the calipers,
Did you open the hydraulic "fluid bleed fitting" when spreading open the calipers wider so the new brake pads would fit ? , or leave it closed and force the Old fluid into the Master Cylinder ??
If you forced the fluid back into the master, this could be your problem, either the mater or a ABS issue.
ABS further complicates doing this on some vehicles.
Try replacing with a NEW master and search for the exact factory method to open the calipers and how to perform a full system bleed to get all of the old contaminated fluid out of your brake system.
Thinking the same MC, to check the booster, very easy, Engine off, apply brake and hold. Start engine the peddle will drop, booster is good.
If the master bypasses, the pedal will sink like that. Most of the time they are tough to ferret out and only bypass now and then.
Start with the simple cheap stuff.
Check the pump and pump to booster hose for cracks. Best to just replace that hose anyway. But for now run your paw over it it see if it has a rip.
Onto the booster. You can do a leak down test with a gauge. Simply install a vacume gauge on the pump line from pump to booster activate the pump then let it shut off. The gauge should hold steady vacume if not the booster or pump have a leak.
Into the master. Fill the reservoir and with someone else pushing the pedal slowly as the truck is running. Watch the fluid level in the front and rear. They should both remain level as the brakes are pressed. If one drops further then the other then a leak or master problem is suspect.
But before you go there we need to look at the proportion valve. Ok these can get stuck during bleeding. Sometimes there is a sliding part in there that allows so much to the front and so much to the rear. This sliding part can get slid one way then you need to go back and adjust it to center. If you reverse bled that can also mess them up.
Watch some youtube videos on the above and try the things out before buying parts.
I say trouble shoot the thing, just blindly replacing parts, creates more problems.
Plus you learn more about how a system actually works.
In the industry that is known as cherry picking.
If ain't broke do not fix it.
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