New Master Cylinder - brakes staying on

[Smokey5159]

Hi all,

Replaced the Master Cylinder today on my 2003 CRD Cherokee, and now the brakes are continually partially on and the pedal has no free play at all..! As soon as the foot touches the pedal the brakes bite.
What has changed? It was just a simple swap and the MC was bench bled and slotted in well. I'm at a loss as to why.
Is there an adjustment on the pedal?

[Macca2801]

Check the depth of the mounting on the booster to the pushrod and then compare to the mounting face on the master cylinder to the piston step. The difference should always be a clearance not a preload or the compensation port in the matster cylinder will stay closed and hold pressure in the brake lines.

[DarrenR]

You have air in the ABS unit, the solenoids need to be bled/reset for it to function.

I manually bleed brakes as normal, then connect the scan tool and bleed the abs unit, see how it feels, then bleed again manually if required.

So, you need a scan tool to achieve this.

Regards
Daz

[bleetnblurt]

Originally Posted by Smokey5159

Hi all,

Replaced the Master Cylinder today on my 2003 CRD Cherokee, and now the brakes are continually partially on and the pedal has no free play at all..! As soon as the foot touches the pedal the brakes bite.
What has changed? It was just a simple swap and the MC was bench bled and slotted in well. I'm at a loss as to why.
Is there an adjustment on the pedal?

did you by any chance check the reg valve in the booster , that funny black plastic thing where the vacuum pipe goes to ?
did you bleed the brakes correctly ?
starting at the rear , then the passenger front then the drivers ?
and doing so while the engine is not running ?
did you bleed by bleeder bottle , or by someone helping you to pump up , and then hold all the way to the floor and quickly closing the bleeder ?
last time I did I did it without starting the engne bled all way round then started the engine (let the ABS pumping system go through the cycle) and then bled the same order again .
but really sounds like the booster is in overtime (or steroids)

[Smokey5159]

Guys - thanks,

A little more info...
I didn't/couldn't bleed the brakes, as the bleed/nipple screws were so corroded in and my spanner started rounding the nut. I then assumed my Master Cyl swap was so clean that no air got in, as it was a nice rock hard pedal anyway - so no bleeding took place. Also, I don't have ABS on my KJ, so I think that simplifies it a little.
Haven't been able to look, and will tomorrow armed with your good adivce.
Paul.

[bleetnblurt]

I thought all kj had ABS , if you cant bleed , there is the problem , you need to ,
spray a bit of WD 40 onto it , let it sit , and if they burr , try a set of needle nose vice grips onto them , if you still can not move them , or they break you will need to pull them off and drill it out (carefully ) and remove the thread and flush them and buy new ones.
in any case it is best to buy some new bleeder nipples,

[senatorjohn]

Take note of Macc's explanation.
John