New disc brakes not working

[jcode68]

I recently upgraded my front manual all drum brakes to power assist with new SSBC 4 piston front discs in the front and the original drums in the rear. I did an extensive write-up on the swap in this thread:
http://forums.stangnet.com/showthread.php?t=657372
The reason for the write-up was because there have been questions about fitment on 67/68 Stangs using the JMC hydraulic clutch setup. The good news is it fits (see write-up), the bad news is I can't for the life of me get the brakes to work properly and I'm extremely frustrated. The problem is when I press the brake pedal, the stopping power is extremely weak. I have to use a lot of force on the pedal and it seems like the calipers are not applying much force to the rotor. I can stop, but it's very poor performance to the point I don't feel safe driving the car. The old manual drum brakes performed much better. I also don't think the power booster is working properly and would like to know how to verify it is working? I know something is wrong since this SSBC Force 10 kit is a high performance piece. I haven't called them yet, but will if I can't figure it out with the help from Stangnet. Here's what I have done:

• Ensured the system was properly bled and have a firm pedal.
• Initially used the original distribution block that was in the car, but since replaced it with the small unit that came with the SSBC kit - no change.
• Replaced the master cylinder that came with the power booster I bought private sale (was brand new OEM style with Bendix Booster) with the unit that came with the SSBC kit. The kit I ordered frome SSBC was for manual disc brake since I picked up the power booster seperate. Would this matter?
• I checked the vacuum from the port on the intake that is connected to the power booster and I'm pulling a solid 18 inches.
• To also be clear, part of my swap included changing the pedal to the correct OEM power brake pedal (see write-up).

The only other thought I had was I did not change the steel brake lines that are original to the car. Would there be a difference in size between disc versus drum brakes? I'm stumped and would really appreciate some ideas.

 

[Eyeless_Seven]

Have you checked the rod to make sure you are using the correct lenght one for this application?

 

[jcode68]

The rod on the back of the booster is connected to the booster and then connects to the brake pedal. The rod between the booster and MC is the one that came with the booster/MC I purchased and is supposed to be OEM. I think it is adjustable, what are the steps to verify this rod is adjusted properly? Also, wouldn't that just result in more pedal travel versus poor braking performance?

 

[DukeGnarley]

some master cylinders have a deeper hole for the pushrod. make sure that the rod between the booster and the mc is the correct length. It sounds like it probably is. some brake pads need to be warmed up before they work properly as well. Also, if you haven't already (i didn't read your other post) make sure you get some brake cleaner and get those discs completely clean.. sometimes they come coated with something to keep them from rusting.

 

[jcode68]

some master cylinders have a deeper hole for the pushrod. make sure that the rod between the booster and the mc is the correct length. It sounds like it probably is. some brake pads need to be warmed up before they work properly as well. Also, if you haven't already (i didn't read your other post) make sure you get some brake cleaner and get those discs completely clean.. sometimes they come coated with something to keep them from rusting.

Thanks. I did clean the rotors well with brake cleaner. I checked my service manual and they show a procedure to check the length of the rod, so I'll add that to the list. Given how poor the performance is, I can't believe it's due to pads not worn in. One thing I noticed was the check valve on my power booster has 3 hose connection ports. Does it matter which port I connect the vacuum hose to? Another thing I will check tomorrow.

 

[2nd Mustang]

Is the brake pedal "hard" or soft with the engine running? A hard pedal means the booster isn't working.

 

[rbohm]

first make sure you have the right master cylinder, and not just an oem style. you need one with the right bore size. an m/c with too large a bore size will send plenty of brake fluid to the calipers, but not enough pressure. you should have a 7/8" or 15/16" bore mc. remember that disc brakes work on pressure multiplication far more than drum brakes do.

second, if you didnt bed the pads in properly you may have glazed them and you wont get much stopping power from them. also if you got the wrong pad material you wont get much stopping power untill they get to the proper temprature. full metalic pads require far more temprature than semi-metalic or organic pads do. race oriented pads also require more temprature than street pads do.

and as stated, if the power booster isnt working, then you need to stand hard on the pedal to get any real braking power as you have a large return spring as well as the diaphram to move, and they both take more effort than if you had manual brakes(drum or disc).

 

[Rusty67]

Check the 1 way valve on the booster and make sure it is opperating properly.

 

[jcode68]

2nd Mustang - the pedal is soft when the car is running and stopped. This is why I feel the booster isn't working properly. Seems to be no difference at all. Once the pedal reaches a point, it firms up and engages the calipers, but it's weak.

first make sure you have the right master cylinder, and not just an oem style. you need one with the right bore size. an m/c with too large a bore size will send plenty of brake fluid to the calipers, but not enough pressure. you should have a 7/8" or 15/16" bore mc. remember that disc brakes work on pressure multiplication far more than drum brakes do.

I'm using the MC that came with the SSBC kit, which is specific to a 68 Mustang. I will check the bore size in the spec sheet.

second, if you didnt bed the pads in properly you may have glazed them and you wont get much stopping power from them. also if you got the wrong pad material you wont get much stopping power untill they get to the proper temprature. full metalic pads require far more temprature than semi-metalic or organic pads do. race oriented pads also require more temprature than street pads do.

I haven't driven it enough to glaze the pads and I haven't been comfortable enough to properly bed them. It really feels unsafe.

and as stated, if the power booster isnt working, then you need to stand hard on the pedal to get any real braking power as you have a large return spring as well as the diaphram to move, and they both take more effort than if you had manual brakes(drum or disc).

I really think this is where the problem lies. I have a check valve with 3 ports on it. 2 are capped and the other I'm using to connect to the intake vacuum port. Does it matter which port is used for the booster? What other methods can I use to test the booster for failure? Thanks for the help guys!b

 

[jcode68]

Ok, have some more info after spending a few minutes in the garage. I hooked up a Mighty Vac hand pump to the booster and was really only able to get the needle a little above 5 (not sure if the scale is PSI or what?). The vacuum would hold and then start to drop very slowly. Is this the way it's suppose to work or should the Mighty Vac be able to go higher than 5?
Secondly, with the engine off, my brake pedal travels approx 4-5 inches before it starts to firm up. Does this indicate the rod between the MC and booster is too short or does it indicate a problem with the booster? I really don't know how the booster inners work, so I'm flying blind. I really need to know how to diagnose the booster for problems. The check valve does not pull out, at least not easily which leads me to believe there is probably a clip inside holding it in. How can I verify the check valve is working properly? I really want to have a game plan for troubleshooting this before I start ripping it out again. All help is appreciated.

 

[6T6coupe]

I think the rod from the booster isn't engaging your master cylinder until you have it pushed pretty far. That would account for the lack of boost and the soft pedal. The hole in the MC is too deep. A couple of solutions come to mind:

1. Make the booster rod longer. It should be threaded and you can make it longer. The trick is to not make it too long or you risk having the MC engaged all the time with the compensating port closed.

2. add a spacer rod into the MC. You can drop in another piece of rod to take up the free space. Again, getting that length right is important.

3. Get a differnt MC. There are other MC's that have a shallow indent instead of a deep hole, mostly GM applications. Look for a mid eighties mid size GM car and look at the pushrod hole.

 

[jcode68]

Was a bad booster

Thought I would update this thread with some closure on my power booster problems. After much troubleshooting and a few phone calls to JMC Motorsports, I ended up sending the booster back to JMC. Upon their inspection, they determined the internals were "ripped up" and essentially the booster was junk. Not sure what caused this since after installing, I put less than 3 miles on the car while I troubleshot the problem. The long and short of it is JMC sent me a replacement with the MC attached and the rod adjusted. I bolted it in, bled everything and went for a test ride. Wow, that's what power brakes are supposed to feel like! The SSBC front discs work unbelievably well. In fact, my 68 now stops better than my 2000 GT with less effort. It was unfortunate that I wasted as much time as I did troubleshooting the power booster, but them is the brakes (pun intended).