Front disc conversion woes

[slumbers]

Greetings from the UK !

My 68 Charger originally had power drum/drum brakes and I have replaced the fronts with a  Wilwood 4 pot Disc brake kit . I have retained the drum brake booster and purchased a Dorman M 71258 disc /drum master cylinder . It still has the original valve assembly with brake light switch below the master cylinder but I have fitted an adjustable proportional valve in the rear brake line . After bleeding the brakes several times the front brakes seem to have a hydraulic lock after two pushes of the pedal and stay hard on . I have checked that the piston in the m/c is not held on by the rod from the booster and  also unbolted the m/c and moved it away from the booster to be sure. Would a faulty brake valve cause this ? The brake warning light is not on . The only way to turn the front wheels is by releasing brake fluid from at least one cylinder at each front calliper. The rear brakes have been bled a couple of times successfully so there doesn't appear to be any sort of problem at the rear ......

[375instroke]

Old brake hoses can cause this.  I assume with a conversion they are new?  Do they release if you open the brake line at the master?

[slumbers]

The Wilwood kit comes with new braided hoses....but I will try releasing the pressure at th M/C and let you know.😎

[slumbers]

Ok , When the front brake line is loosened at the M/C the front brakes release. Also when the engine is running and you have servo assistance the brakes just about stop the car rolling on idle but the pedal bottoms out ......  

[303 Mopar]

Did you bench bleed the M/C first?

[slumbers]

Yes I did !

[Mopar Nut]

Did you adjust the pushrod in the booster?

[slumbers]

I adjusted the pushrod to be about 1/16" less than the depth of the m/c hollow shaft so that the m/c piston is not actuated when the pedal is returned .

[Mopar Nut]

That master you are using is a 1" bore, better to use a 15/16 bore for a softer pedal. That would not cause the hydraulic lock though. If your booster don't look like this, you have the wrong booster.

[John_Kunkel]

Also when the engine is running and you have servo assistance the brakes just about stop the car rolling on idle but the pedal bottoms out ......  

I can't think of anything but a faulty MC that would cause the pedal to bottom under boost. Don't know how that relates to the front brakes not releasing but it could.

[garner7555]

Also when the engine is running and you have servo assistance the brakes just about stop the car rolling on idle but the pedal bottoms out ......  

I can't think of anything but a faulty MC that would cause the pedal to bottom under boost. Don't know how that relates to the front brakes not releasing but it could.

I agree.  That's the only thing that makes sense.     

[Charger RT]

pull the vacuum check valve out of the booster when the brakes are locked up and see if they release. If they do I would think the booster is bad.
Tim

[slumbers]

pull the vacuum check valve out of the booster when the brakes are locked up and see if they release. If they do I would think the booster is bad.
Tim

Hi Tim , I have already tried that , nil fix 

Just another thought ,the first master cylinder I used brand new from summit would't bench bleed the rear brake port, I took it apart eventually and found a seal missing from manufacture ! Summit sent me a replacement,this one , maybe I should look inside ........

I have asked Summit Technical Dept for advice too.

Will let you know how I get on .

[slumbers]

Well an update , Thanks for all the suggestions !  I found that the new master cylinder piston assembly was sticking in after you released it , was taking up to a minute to return when bench bleeding and sometimes remained 1/8" short of the rear stop !! Even after lubing the bore it did'nt get any better so I put it in the other master cylinder body I had with the missing seal and it seems to work OK . No more sticking . I found I had adjusted the servo pushrod on the short side DOH !   ,  but even after sorting that  with the drum brake servo you appear to run out of pressure with the brakes as suggested by Mopar Nut . I found on another forum that the ratio is incorrect and a dual servo is required with disc /drum . So I will order a new servo with matching master cylinder just to be sure  

https://www.forbbodiesonly.com/moparforum/threads/68-b-body-brake-booster-differences.28216/

[slumbers]

New 8" dual diaphragm servo and matching master cylinder fitted from "Pirate Jack "
http://www.ebay.co.uk/itm/1966-70-Charger-8-Dual-Power-Brake-Booster-Master-OE-Bendix-Style-/161544920345?hash=item259cd3dd19:g:

     Booster fiddly to fit but managed to do it with the dash fitted , removed a blank switch cover and with 4 x 6" wobbly 3/8 drive extensions and a couple more to take my ratchet past the steering wheel took 20 minutes with drivers seat removed !! Had to grind some metal from bulkhead to get the booster to go in with a rotary file . This was actually the most difficult bit ,needs about 1/8" removed each side of the opening .


Instant fix on the brakes , they pull up very well .

Thanks everyone for all the advice .....👍👍

[taxspeaker]

Thanks for keeping us up to date on this one.

[tcurtis]

Did you have any issue with the lines coming out of the master cylinder on the driver's side?  I'm looking right now at the exact same unit from Pirate Jack or the same unit with lines coming out of the passenger's side from Moparts.  There is an approximately  $180.00 difference in price (Pirate Jack cheaper) and the location of the hydraulic lines seems to be the only difference.  For that difference in price I'm willing to sacrifice a bit of authenticity to get back on the road.

[slumbers]

Just seen this , I bought the master cylinder with the brake pipes on the drivers side . I also ordered a new set of correct pipes for a disc/drum master cylinder from the brake valve block which are a little longer than the drum/drum master cylinder and drum servo set up although the original pipes would have reached but been a little tight .

[375instroke]

What check valve did you use?  The booster I got came with a single nipple fitting, but Chargers need two to operate the headlight doors.