Brake light on, locks up rears

[NDT]

Newly arrived M1025A2. Master cyl full, no leaks, stops on a dime but it feels like rear axle is doing the work, locks up tires rear only with hard application. Any special HMMWV wisdom here other than bleed brakes and replace master cylinder if problem does not resolve?

 

[simp5782]

I remember the old rubber Chevy brake hoses would deteriorate internally and allow flow to go to the caliper but would basically lock it up and wouldn't let the pressure release back keeping a caliper semi or fully locked. Also seen the latter that it wont allow fluid to reach the caliper causing a lockup due to the porportioning

 

[Coug]

It also depends on the amount of weight in the vehicle. The rear end typically doesn't have as much weight on it, so the rear brakes lock up before the front do, especially when applying the brakes hard. I can make the brakes lock up on the rear of my M1123 even with a small load in the back when braking hard. With it loaded near max capacity it should brake more evenly.

If the nose dips down when braking the front are doing their job. If it feels like the nose is lifting at all then the front aren't working. HMMWV suspension is a great deal stiffer than a lot of civilian vehicles, so might not be as noticeable as a regular car.

You said it stops on a dime. If it's stopping quickly then the fronts are doing their job, otherwise you'd be dragging the rears everywhere whenever you applied the brakes, as weight shifts forward when you brake, so there would actually be less traction for the rear tires.

Remember these are inboard brakes, so the shafts absorb some of the rotational energy when braking, and give it a different feel than almost every other vehicle out there.

 

[papakb]

Front brakes are meant to do about 60% of your braking so if the rears lock up and the fronts don't something's going on. There's a proportioning valve there by the master cylinder that could be locked in one side causing an uneven distribution of braking. Pretty easy to take apart to check but it means bleeding the brake system afterwards. If it comes down to replacing front calipers they're '87 Eagle calipers and can be bought new with no cores for under $20.

Removing #9 gives you access to a ball bearing that moves back and forth to proportion the braking. If it jams it'll hose your braking. The sensor just above it is magnetic and should keep the bearing centered.

 

[juanprado]

My experience in another world selling parts is that is a classic symptom of the portioning valve not working properly. Does the master cyl have a lot of brown sludge on the bottom? If so a flush, fill and rebleed might help?

Physical inspection of brake hoses? Remove calipers from mounting and see if piston retracts easily with c clamp?

If rear are the emergency type is the cable working properly/adjusted so as not to drag or bind? Is the pistons moving properly?

 

[Bulldogger]

The brake light on the dash or rear brake lights? It sounds like you mean the dash light. I'm only familiar with the M998, and the M1025A2 might be different but if it's the dash light it means the parking brake is engaged...

Your mention of rear brake issues makes it also sound like the parking brake assembly is jammed or something. I'd open up the boot underneath and give a good look-see in there. You can visually verify the parking brake is letting go as well, back at the rear calipers.

My apologies if the 1025 is noticeably different from my 998 in this respect.

Bulldogger

 

[Retiredwarhorses]

Brake light on dash comes on with low pressure in the proportioning valve....

 

[papakb]

Here's the schematic for the proportioning valve. Anytime the ball bearing moves forward or backward due to a pressure imbalance in the system it puts a ground on the warning LED on the dash and lights the light. That LED is a dual purpose unit telling you either the E-brake is on or there's something wrong in the brake hydraulic system. It's the same circuit for all variants.

 

[NDT]

Thanks everyone so far. Update, I unplugged the proportioning valve connector and the brake warning light did not extinguish. Sure enough it’s a stuck handbrake switch. Based on ya’lls responses, with a completely empty truck, this might be the nature of the beast.