Just wanting to check all the basics - did you bench bleed the master cylinder to make sure all air was removed? Are you all disc or disc/drum? (also do you have a power brake light switch?)

Definitely sounds like air in the system.  How did you bleed them?  I have found that to properly bleed a new system you need to bench bleed the MC first.  Install the MC, then follow the proper bleeding order (see below).  I only bleed brakes using a piece of clear vinyl hose over the bleeder running into a cup with enough brake fluid in it to cover the end of the hose.  Open the bleeder and pump the brakes until there is no air.  Close that bleeder and move on to the next one in the sequence.  Do that once through, then do it a second time.  You can probably only go 15-20 pumps at a time before needing the refill the MC.  You can't let the MC get too low or you introduce air into the MC and will have a heck of a time getting that bled all the way through the system.  This is how I've bled brakes for 20 years.  It always works, and works well.  You will need an extra quart or so of fluid.  That's about the only downside to this method, but fluid is cheap, while good brakes are priceless.  

What order did you use to bleed them?  The order should be right rear, left rear, right front, left front.  Bleeding out of order can make bleeding the system properly impossible, like if you bleed the fronts first.  This is a peculiarity I've noticed only on new systems, and why it never seems to be a problem if you just hang a caliper on a car that needs one.  

Another thought is that sometimes it helps to tap the calipers with a plastic hammer with the bleeder open and connected to the cup as noted above.  This helps free air bubbles that may be hiding.  

You indicate that the car has a B&M blower, what type power booster are you using? 

There's no need to run a residual pressure valve on a disc/disc system.  However, this makes me wonder what master cylinder you are running?  Is it for a disc/disc setup or a disc/drum setup?  Disc/drum masters often have a residual pressure valve built in for the rear drums.  

Then there's master cylinder sizing.  I wouldn't guess at the size.  If its 1" that's different than, believe it or not, 15/16" in terms of performance.  The master needs to be matched to the system.  You can check this by looking at the relationship between the front disc piston apply area and the apply are of the master cylinder bore taking into account your pedal ratio.  

I’m back! Ok the front are Granada type calipers. Bleeders are on top pointed to the rear. Rear calipers are the type MS mentioned that are adjusted by the parking brake (which I do not have connected). I adjusted the rear pistons to be almost touching the rear rotors. I bled the system for the 9011th time by having my GF pump the brakes and hold. My symptoms are still as follows- pump the brakes they feel great stationary in the garage- hold the pedal and they stay up and feel normal for as long as you care to hold them.  Let off and press again and they nearly go to the floor. Pump them again till pedal feels good and start the car- straight to the floor. The booster and master are units I bought from this site 10 years ago or so and never had fluid until just a few weeks ago- maybe bad seals? I did test the booster by shutting the car off and waiting 15 min then removed the vacuum line- it had held a vacuum. I also opened the adjustable prop valve wide open to bleed the brakes. I’m at a loss on what it is.

Last edited by Brents65 (5/13/2026 5:18 PM)

Mabe I missed it, but if you’re running a blower, where are you getting a vacuum from? 

Re-read your response in post 4 and start there.  At this point I'd start over with a new master and follow their recommendations on bench bleeding.  If you don't bench bleed, there is no way to get any remaining air out if present.  Because you may have gotten sway without bench bleeding in the past may not apply in this case.  If that doesn't do it, consider 2 psi residual pressure valves.  Others may not have used them but that doesn't mean they won't work in your situation with your calipers, etc.  Do what you have to do to make your system work.

I have the CK-165M (CABLE CLUTCH & POWER BRAKE RETROFIT
For 65 - 66 MUSTANG) on my ‘65 since 2017.

I am using 2001 Mustang master cylinder (V6 with traction control 10mm and 12mm ports).

I used the above with Granada brakes and proportioning valve and stock rear drums initially, worked great.

When I switched over to 2016 Mustang GT brakes front and rear, a Wilwood in-line proportioning valve was added to rear circuit.

I made my own rear lines that T’d off the stock rear brake line.

Since my lines were modified and brake system was opened up for modifications, I gravity bleed my entire system at each and every junction.

It was worth the time spent.

I also made the “parking brake “ functional as well.

The rear pads do contact the rotors as TKO mentioned.

Is it possible that air might be trapped in lines?

I am not familiar with the B&M super charger and where the vacuum should be tapped for the brakes.

When you mentioned the booster still having vacuum on it is meaningless…kinda like saying there gas in the car…um…how much?

The vacuum at booster needs to be monitored with a gauge to see what is happening in the system.

The ‘65 had 4 wheel power drum brakes from factory.
I have been very satisfied and impressed with the MustangSteve power brake booster setup for my use as a summertime daily driver. It was worth the time and effort.

A tip for the Granada brakes, use the semi-metallic pads for police/taxi use.

Last edited by Nos681 (5/16/2026 7:12 AM)

Just for clarification, which master cylinder are you using?

https://fyi.boardhost.com/viewtopic.php?id=9258

Back in 2018 you mentioned the disc/drum master with your kit.

I have used the 2001 (2000) v6 master cylinder with Granada brakes/stock drums and currently with 2016 GT brakes front and rear with absolutely no problems.

Nos681 wrote:

Just for clarification, which master cylinder are you using?

https://fyi.boardhost.com/viewtopic.php?id=9258

Back in 2018 you mentioned the disc/drum master with your kit.

I have used the 2001 (2000) v6 master cylinder with Granada brakes/stock drums and currently with 2016 GT brakes front and rear with absolutely no problems.

I too am using the 2000 V6 master cylinder on a stock Bendix booster with Granada brakes in front and V6 disc brakes in back. 
 

Nos681 wrote:

Just for clarification, which master cylinder are you using?

https://fyi.boardhost.com/viewtopic.php?id=9258

Back in 2018 you mentioned the disc/drum master with your kit.

I have used the 2001 (2000) v6 master cylinder with Granada brakes/stock drums and currently with 2016 GT brakes front and rear with absolutely no problems.

 
I believe it is the 2000 v6 mustang unit. I bought the kit from MS a long time ago. I turned the prop vale all the was in shutting off the rear brakes and stopping is still 75%. No lock up.

Back when I had the 2001 V6 master cylinder, 13" Cobra brakes, and a Booster Dewey rebuilt booster, I could only get 925 psi to the front calipers which would not lock up the brakes. I removed the booster, changed brake pedals, and the brakes have worked perfectly since. Because of the increased leverage of the manual brake pedal, locking up the tires is quite easy. I have never once wished I still had power brakes. More room in the engine bay is an added feature.