Braking problem

[BelgiqueB]

Refurbished the braking calipers on my 2003 dodge ram and ever since I've had a soft pedal when the engine is running. The brake pedal is firm with the engine off but as soon as i turn the engine on the pedal goes to the floor with very little effort, while driving the brakes work , i can lock them up but the feeling in the pedal is off, not firm and progressive like usual. It's soft and then all of a sudden the brakes lock.

The car has been converted years back from a vacuum brake booster to a hydraulic brake booster running of off the power steering pump, system worked ok before refurbishing the calipers

Things I've done so far:
- bled, rebled, re re re bled the braking system, even bought a Sealey brake bleeding kit that puts the entire system at overpressure from the master cylinder reservoir to push through the oil. I've ran multiple liters of braking fluid through the system and there is just clear oil coming out, no bubbles.
- checked the entire system for leaks with the brakes engaged, no leaks to be found
- swapped out the master cylinder and hydraulic brake booster for a new one, the master cyclinder was bench bled before installing in the car.
- checked the power steering pump hydraulic pressure, was around 110 bar when engaged which seems to be normal
- checked all calipers for correct orientation, all the bleed nipples are at the top of the calipers

All to no avail, when i press the brake pedal with the engine turned off it's firm and solid as it should be, but as soon as i turn the engine on the pedal goes straight to the floor without a lot of effort.

I'm all out of ideas, hoping someone here might have the answer :-), thanks in advance.

 

[Wildefalcon]

I'm guessing brake pedal adjustment is causing the boost to be "on" more than it should?

 

[dobbslc]

It would be worth checking how far the pistons retract once the peddle is lifted.

 

[MetalMonkey]

Does it have ABS?

 

[Erie Fred]

I know you wrote that you bled the system, did that include the rear brakes as well ?
Does this have ABS ?

 

[james butler]

bulging flexis?
have had it in the past where brakes work but pedal is spongey
get someone to press the brake and go around checking all flexis for bulges or distortion

 

[BelgiqueB]

Car does have ABS, did lock the brakes multiple times when driving on a gravel road to cycle the abs system and release any trapped air.

Checked all the flexi lines for bulging with the system under pressure and found no issues.

All 4 calipers have been refurbished and bled as per manual (pass rear, driver rear, pass front, driver front)

 

[BelgiqueB]

It would be worth checking how far the pistons retract once the peddle is lifted.

Will do, but how would this affect brake pedal feel?

 

[dobbslc]

The peddle will sink a long way before you get any braking as the pistons have to move much further.

 

[BelgiqueB]

The peddle will sink a long way before you get any braking as the pistons have to move much further.

Makes sense, I just checked all four calipers and the pads are in contact with the disc with the pedal lifted. Need a bit of effort to make the discs spin even.

 

[dave h 2]

when you refurbed the calipers were any of the pistons reluctant to move?

 

[MetalMonkey]

Car does have ABS, did lock the brakes multiple times when driving on a gravel road to cycle the abs system and release any trapped air.

I'd double check on a vehicle specific forum/discussion group, or in the workshop manual on this - cycling the ABS system like that doesn't necessarily do anything, and bleeding the ABS module on some vehicles requires the use of a scan tool.
Some systems just bleed conventionally, and some are complete nightmares for no obvious reason.

I'm not familiar with that vehicle, but if its a common problem I guarantee you wont be alone - if it isn't a common issue then you can look elsewhere - but that would be my first port of call.

 

[ajlelectronics]

If you suspect air (I don't) a useful trick is to leave the brake pedal wedged down for 24 hours. Not sure how it works but it is often effective at getting the last bubbles out.

 

[RWD3M]

Discs or drums at the rear?

 

[ajlelectronics]

It calls out "faulty servo" to me.

 

[the snooper]

Sometimes when bleeding a system people push the pedal all the way to the floor and into the unworn part of the master cylinder, for whatever reason I've known this to damage master cylinder seals, without the engine running place pressure on and off the pedal, jiggle it if you get me and see if the pedal creeps down to the floor

 

[jo1010]

Did you bleed abs unit separately as well? Sometimes you need to and then re-bleed the brakes again. Otherwise I'd go for damaged mcyl.seals

 

[m_c]

The tolerance on the caliper piston seals is quite critical, and I have had problems with a dodgy refurbed caliper in the past (have I ever mentioned that ECP owned brand parts are rubbish?), but that just resulted in a very soft pedal.
You could see the piston being retracted more than normal when the pedal was released, however your comment about lack of progressiveness and the brakes still locking, albeit suddenly, would suggest it's more likely to be something else.

As has been mentioned, it could be a master cylinder issue.
If the seals have partially failed, they can seal under pressure, but leak under partial pressure.
With the engine off, gently press the pedal, and slowly increase pressure over a minute or so, and see if it starts creeping down.

Incorrectly bled/failed ABS modulators can also cause strange brake pedal behaviour.
You'd have to find the official bleeding procedure, as although some don't need any special bleeding, some require purged via diagnostics to get any trapped air out.

 

[ajlelectronics]

Incorrectly bled/failed ABS modulators can also cause strange brake pedal behaviour.
You'd have to find the official bleeding procedure, as although some don't need any special bleeding, some require purged via diagnostics to get any trapped air out.

Reverse bleeding to the modulator has twice saved the day for me.

 

[tom2207]

Id be bleeding it again ,,, just as a starting point