Strange lack of power after brake repair

natsys

Seasoned Expediter
Owner/Operator
This one has me baffled.

I replaced both front calipers back in November on my 2011 Hino 258 LP (juice brakes). They weren't the same brand (long story), but this one was a reman Centric. Monday the brake line connecting the two halves of the piston let go--too close to the wheel and it wore through the line. Got some new line, made up a replacement and bled the caliper. It took about 50 oz of brake fluid since I lost so much. I did have an assistant help me bleed the brakes by pumping the pedal and did it with the engine running. I bled both fronts since so much fluid was lost in that circuit. It shot out of the collection bottle I was using under extreme pressure and got all over everything.

Went out to test drive it and it drives like its derated, just crawls along. Won't exceed 1200 rpm--until it passes 18 miles per hour and then it's normal. This is a real pain to drive in stop and go traffic, especially if I'm heading uphill. I checked the brakes, nothing sticking. I cleaned out the ABS sensor plugs with electrical cleaner--didn't help.

If I start the engine and just drive , it runs fine until I stop. Then it won't exceed 1200 rpm. If I put it in neutral or reverse and then back in drive, it will work fine. If I drive even a few feet and then stop (below 18 mph) it will "derate". There's no check engine or abs light on. I haven't hooked up DX2 yet as I'm hoping I can figure out what I may have accidentally affected or someone else will know. It never behaved like this before.

Any ideas?
 

natsys

Seasoned Expediter
Owner/Operator
Well, not sure why this problem existed, but I did get it to go away.

I seemed to remember that when I changed the front calipers originally I could hear my helper a lot easier when bleeding the brakes so the engine probably wasn't running. Thinking that might be the problem, I re-bled just the right side front calipers with the key on, engine off. Only two shots because I needed more fluid to replenish the reservoir. Wasn't looking forward to driving to my stops that day, but the 1200 rpm power limit never happened again.

I'd still like to know how an imbalance sensed in the hydraulic brake system 1.) doesn't trigger the brake warning light, and 2.) influences the transmission or engine to reduce power. I know the abs sensors can cause the transmission not to lock up so perhaps it's something associated with this.

I'll still re-bleed both front lines to make sure I have all the air out, but for now seems like problem solved.
 

greasytshirt

Moderator
Staff member
Mechanic
Sounds like a brake switch correlation issue. Make sure the plunger of the uppermost switch is fully depressed with the pedal released. Twist it counterclockwise a quarter turn and it will pop free, the push it in until the plunger is depressed and turn it clockwise to lock it back in.
 
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