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10/18/2020 9:48 AM  #1


Brakes won’t lock up

I have a 65 coupe with 302 mustang Steve’s PB five brake booster conversion Maverick double bowl disc drum master cylinder proportioning valve set up in the rear line  factory rear drum brakes front disc conversion Wilwood four piston dynalight conversion

Brakes work fine under normal driving but if I have to panic stop the brakes will not lock up I have adjusted the rear brakes rebleed the entire system to make sure there’s no air open the rear proportioning valve all the way but still cannot get the brakes to lock up under panic or hard stop does anyone have any ideas what could be wrong

Thanks

 

10/18/2020 11:02 AM  #2


Re: Brakes won’t lock up

What are the engine mods and at what elevation do you live?  I had a similar problem with the MS PB setup with the four cyl. Fox booster, 08 GT 12.5 front discs and 11+- Explorer rear discs.  At my elevation (6K) the idle manifold vacuum was only 9.5 inches.  I added a mechanical vacuum pump which brought the running vacuum up to 17.5 inches and that helped but brakes still did not perform up to my expectations.  Finally I swapped to a Fox V8, dual diaphragm booster and that did the trick.  My 66 now has REAL brakes.

BB1


"you get what you pay for, good work isn't cheap, and there are NO free lunches...PERIOD!"
 

10/18/2020 11:21 AM  #3


Re: Brakes won’t lock up

Engine has no modification stock rebuild

     Thread Starter
 

10/18/2020 3:12 PM  #4


Re: Brakes won’t lock up

I would check the system pressure, my guess is its insufficient.  I suspect a mismatch between the master cylinder and the calipers. 

 

10/18/2020 4:01 PM  #5


Re: Brakes won’t lock up

The 15/16” master cylinder should provide correct pressure for the Wilwood calipers.  My neighbor is a factory rep for Wilwood, and we had a discussion about their Mustang replacement calipers.  He said their design replicates fActory pistob area and pad area.  The improvement over stock Mustang calipers is that they have better piston seal design that does not freeze up.

From my experience with modern pad material, it is harder than the old stuff most people are used to and just does not stop as well.  Add that to the fact that most installers do not bed the new pads, but instead immediately head out to do a high speed panic stop with their new disc brake system, and you get a fried pad that needs to be replaced in order to ever stop properly.   If that describes your case, at least remove the pads and block sand them on a glass pane with 80 grit sandpaper. Then reinstall and properly bed the pads.  (Google it).


Money you enjoy wasting is NOT wasted money... unless your wife finds out.
 

10/18/2020 4:08 PM  #6


Re: Brakes won’t lock up

i will try that but i did bed the pads as there instructions said

     Thread Starter
 

10/18/2020 4:29 PM  #7


Re: Brakes won’t lock up

The high end pads in particular don't stop as well because they are designed mostly to last a long time.  The cheap pads seem to work better, but have less pad life.  New high end pads and old rotors is also often a mismatch because the newer rotors seem to be softer.  The old rotors are hard and new pads are hard and they just don't create enough friction.  Rotors on new vehicles are designed to be consumable.  This is why almost no one turns them anymore; there's typically no point.  They have enough meat for the pads to wear them out and that's it.  By the time you need pads you need rotors too. 

That's all a bit academic.  If you have the correct pads and rotors I don't think that's your issue, especially if you bedded them in.  What it sounds like to me is that the calipers are just not exerting enough force on the pads when you really stomp the pedal.  Its fine for normal driving, but when you get into the last bit of pedal travel the pressure doesn't continue to rise.  My thoughts are maybe the master is bad.  Sometimes you get corrosion in the bores in areas where the piston doesn't typically travel.  You bleed the system and the piston can now go into that area of corrosion, a seal gets nicked, and this is the result.  I've had this issue several times. 

 

10/18/2020 4:44 PM  #8


Re: Brakes won’t lock up

I don't think the Maverick M/C has a residual pressure valve. An inline 10# valve in the rear line may help ... it wont hurt it.


"Those telephone poles were like a picket fence"
 

10/19/2020 5:19 AM  #9


Re: Brakes won’t lock up

My understanding of residual pressure valves is that they are for drum brakes.  It keeps a small amount of pressure on the brakes so the brakes grab sooner.  They are also used when the master cylinder is not the highest point in the system.  With a disc system where the master is the highest point in the system I don't think a residual valve would help his issue.  The brakes seem to work correctly under all conditions other than a panic stop.

 

10/19/2020 4:32 PM  #10


Re: Brakes won’t lock up

That is also my understanding TKO. Mark has drum brakes in the rear.


"Those telephone poles were like a picket fence"
 

10/19/2020 5:34 PM  #11


Re: Brakes won’t lock up

I recently did the upgrade to power disk with the kit from kentucky mustang and the fox body booster kit from MS with his swing arm all in the front of my 65 and said they would not lock up. I found that after a 1000 miles on them i'm glad they don't because as I found out, when I came over the pass at 70 and saw brake lights. They will toss you out the windshield now without skidding a tire (all 4 corners have 225 x 60 x 15 TAs). they mite skid but i don't need to slam them that hard and you are stopping, stopping very quickly.  That tells me I have more control of the car. I am very happy with the conversion. 

Last edited by Cab4word67 (10/19/2020 5:35 PM)


Just when I thought I wasn't spending $$$ The gremlins came back
 

10/19/2020 6:16 PM  #12


Re: Brakes won’t lock up

I always  "heard" once the tires lock-up and start to slide.......that's the end of your braking. Car should  come to a stop a lot quicker with brakes clamping butt.....NOT locking up. (we're not even talking about the loss-of-control with locked up and sliding tarz)
jus say'in'
6sal6


Get busy Liv'in or get busy Die'n....Host of the 2020 Bash at the Beach/The only Bash that got cancelled  )8
 

10/20/2020 5:40 AM  #13


Re: Brakes won’t lock up

50vert wrote:

That is also my understanding TKO. Mark has drum brakes in the rear.

So he does, I thought he had a 4 wheel disc system.  Probably should have read the initial post a bit more thoroughly. 
 

 

10/20/2020 5:42 AM  #14


Re: Brakes won’t lock up

6sally6 wrote:

I always "heard" once the tires lock-up and start to slide.......that's the end of your braking. Car should come to a stop a lot quicker with brakes clamping butt.....NOT locking up. (we're not even talking about the loss-of-control with locked up and sliding tarz)
jus say'in'
6sal6

That's true, but they need to be capable of locking up.  The best braking is that point just before lockup (threshold braking), which is what ABS seeks to simulate (and mostly fails at). 
 

 

Board footera


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