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Brake Calipers Drag - Ford/Dayton on 1996 Beaver/Magnum Chassis - They just do that?

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I've been chasing brake caliper drag on this RV for a few years - hoping someone has an actual answer...

TLDR: 1996 Dayton dual 2.88" calipers drag, replaced everything, still drag, what an I missing?

1996 Beaver 36' Diesel Pusher: Mgnum 4-bag chassis, 22,000lbs, Cat 3126, Allison 6-speed automatic, Reyco Granning airbag suspension.

Brake system: Bosch HydroMax (#2239045 2" master, power steering servo brake booster, electric pump backup) remote reservoir mounted about 4’ above the master cyl

Front and rear disk brakes, calipers are Dayton 600-908 dual 2.88” piston, cast-iron, sliding caliper (1996 Ford F700 truck).

Purchased in 2016, used only a few times a year. In 2019 I added a EEZTPMS with pressure/temp sensors. Was driving on a long highway (cruise, no brakes) and the front TPMS temp warnings tripped at 120°F - pulled off and checked with IR temp gun - both front and rear rotors were about 330°F.

Checked brake pedal play, ok, Opened bleeder valves, flow but no excess pressure. Continued the journey (more slowly) no seeming issue other than temp (and likely wasted fuel).

With wheels free can feel the brake drag, pushed the pads back with a big wrench and now free, step on brake and release it's dragging.

Did these things - they still drag:

  • Brake fluid flush and bleed - DOT4
  • Replace master cylinder (2x) Bosch new (not rebuilt)
  • Replaced all 5 brake hoses (in-frame bulkhead fitting - nightmare)
  • Tested brake pressure (electronic sensor) run up to redline - no change in pressure
  • Grind off 0.005" of booster interface to master cylinder
  • Change power steering fluid (Dexron III)
  • Replaced front calipers: Nugeon rebuild (one was phenolic pistons, one aluminum), then again with Delco rebuild - sliding surfaces sliding surfaces greased (copper disk brake grease)
  • Replaced pads
  • Rear rotors turned and look perfect, fronts minimal runout

There is about 2psi static pressure in the brake lines: 1/2 gallon remote reservoir is mounted 2' above the master cylinder, master cylinder is 2' above the calipers

After all that:
Step on the brakes and release, brake drag
Open bleeder, flows but not squirt like there was pressure, still drag (so don't think it's hydraulic issue)
Press pads back in with big pliers, no drag

I have disk brakes on lots of other vehicles, and releasing the brake releases the calipers. This is my first experience with medium truck parts - what am I missing?


Edited by excess_lumens
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  • excess_lumens changed the title to Brake Calipers Drag - Ford/Dayton on 1996 Beaver/Magnum Chassis - They just do that?

Testing for brake drag by lifting the wheel off the ground and turning it by hand is not necessarily a good indicator of brake drag. A little drag is somewhat normal. After a vehicle gets moving any rotor run out or wheel bearing play will usually loosen thing up a little. I would be more interested to know if the rotor are still getting hot after driving. Of course the only way to accurately checking that is to roll to a stop without using your brakes and then checking the temperature of the rotors. If they are still hot then I would be as stumped as you are.

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It might be that even the 2 psi prevent the calipers from backing off? There is nothing to push them out other than the rotational imperfections as Tom said. I have only seen the opposite and had to use residual pressure valves to keep fluid from draining back too much with the master under the floor, at axle level in my 50's pickup. Is there anything that would prevent all the pressure from releasing, like an overfilled reservoir or a plugged vent hole in the cap? I know, sounds too basic. 

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Posted (edited)

First indicator of brake drag was TPMS sensors increasing to 120°F from radiated rotor heat - open road not using brakes.  After all the repairs (multiple iterations) no change - they still heat up on the open road, rotor temp is about 330-350° on a moderate (70°) day at 60mph. Yes, checked temps by pulling off without using brakes.

Hand-turn drag seems more significant than I would expect, but haven't measured.

The reservoir is filled about 1" below top and the cap vents are good. 

Two things I can think of:

  • Somehow all the rebuilt and Delco kits have O-rings that are too thick - so the piston doesn't naturally return.
  • The static brake line pressure from the elevated mater cylinder and reservoir is enough to prevent the caliper from releasing - but then pretty much any of these coaches would have that issue. There is nothing in the docs about only partially filling the reservoir - and if static pressure was the issue I would expect the caliper to release when I opened the bleeder - it doesn't.
Edited by excess_lumens
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Unless there is some slight sideways movement of the wheel/disc, the caliper will not retract by itself when pressure is released. The seals will keep it where it is.  Even with perfect disks, the wheel bearings will always have a tiny play while driving with weight on them. We have played with adding springs, pushing the pads apart before the front wheels hit the ground while doing wheelstand launch at drags for that reason. Not a normal situation though. I still think that the residual could play a role in your case, whatever is causing it. Like Tom indicated, release all pressure. Leave pads where they are. Drive without touching brakes with presumably no pressure to a freewheeling stop, Block the wheels again and check for dragging. Should be none or only at a certain spot. Then you at least know the caliper is not to blame. Easy said, I know.

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I had an old 86 P30 that had a brake dragging problem. It had a mechanical hookup between the pedal and the Hydroboost.

There was a crossover bar between the frame and engine that was rusted(had ball and socket in the ends) and wouldn't release the brakes all the way. Drilled the bar and zerk installed, grease and that ended the problem.


Skip H.

96 Monaco Windsor

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So what I read is after assembly you stepped on the brakes and ALL FOUR DRAG? Then you said you open the bleeder while spinning the wheel and spin it again and it still is dragging?  How about this, spin the wheel in the the forward direction and feel the drag, now spin the wheel backwards... does it still drag? I ask this cause I have had similar issues with disk brakes before. 

There was several things I have tried, One thing you need to understand that on most automobiles the calipers have a square cut O ring on the piston so that they keep a minimal brake pad connection with the rotor, not to keep a heavy drag on the rotor but to stop the piston from retracting which causes gap that makes pedal movement extended, plus it is to keep the rotor cleaned at all times. That said the bigger the o ring the more the grip, I suggest to examine the gap between the booster (air or vacuum) and the master cylinder. You need to know that is a adjusting point, and that if for some reason it has "advanced" its length with that the master cylinder can never retract to its at rest position. for those of us that have had the fun of bleeding a new master cylinder BEFORE installing it on the vehicle know that if when retracting the piston it will suck air back in to the chamber and you will never get the air out, in fact when bleeding that same master cylinder and your using a hose dipped into a clear jar that when you retract the piston you can see a slight amount of brake fluid return back up the hose.

Sorry for the tech breakdown, what this sounds like is your master cylinder is NOT being allowed to return the slight amount of "suck" to help return the caliper pistons to there proper position of at rest. It is a very minimal amount however its a VERY important gap between the booster and the master cylinder, needless to say its VERY important to make sure there is NO air in the hydraulic lines. something best accomplished using a pressure bleeder and often this can be not fully accomplished by just pressing on the brake pedal while someone opens the bleed screw due to now line length, possible ABS module, or abnormal bends in the lines themselves.

Good luck, and motor on...

P.S. I believe the gap only needs to be like 1/8 in. 

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Drag feels quite more than expected both directions, in the reverse direction the caliper rotates slightly in the bracket (by design).

Square o-rings should retract the caliper piston a minimal amount when pressure is released, in this cae they appear not to.

I believe the master cylinder is fully at rest with no brakes applied, as fluid flows freely from the reservoir - the fill ports are closed as soon as the master piston is moved, and I also too off 0.005" from the booster piston that actuates the master.

I've bled a few gallons through this using mainly a vacuum bleeder.

No ABS module.

My two thoughts at this point are:

  • the combined remote reservoir and master cylinder height of 4' provides enough hydraulic pressure to create drag
  • somehow caliper rebuilds and rebuild kit square O-rings are oversized and so tight as prevent the piston from retracting naturally


Screen Shot 2022-04-16 at 10.46.41 AM.png

Re: pedal to hydroboost: on this unit the pivot shaft has a zerk (greased it) and it appears to return the whole way - there is some play at the hydroboost actuator rod.

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