Poor Brake Pressure after front disc install

[jcwhite1288]

Ok so i recently installed a scarebird front disc conversion kit (hopefully not my first mistake) and the pressure was ok with the stock single bowl, but i wanted a stronger pedal so i installed a dual bowl. I bent up some new adapters and small fittings that i got from a local auto parts store, bench bled the MC and installed it. Now i have worse pressure with the dual MC than when i had the single! any ideas on why that is? one or two of the small lengths of tubing has a smaller diameter than the stock tube, if thats possibly a problem.
EDIT: 1966 289 btw

 

[88gt]

do you have proportioning valve or distribution block of some kind?
or did you just split the lines?
how about a photo or two of what you did??

 

[jcwhite1288]

ya i will post a photo after work.. i used a new distribution block for the front brakes, and ran a proportioning valve out of the MC straight to the rear drums.. could my refurbished MC be bad even though its not leaking at all?

 

[phutch11]

You are going through exactly the same issues I went through with my scarebird kit.

I had decent pedal pressure with the 67 dual bowl drum/drum MC I had. In an effort to do better I installed the recommended 74 maverick MC and the pedal pressure got much, much worse.

Then I did some research on the s10 caliper used in the kit. Turns out they are a quick fill design which calls for a specially designed MC.

None of the bleeding, adjusting, and additions of residual pressure valves fixed the issue.

The issue was resolved by using a MC from a s10. The pedal feel is awesome now.

The s10 MC will fit, but you will need to oblong the mounting holes, get an adjustable pushrod, and figure out a way to positively retain the pushrod.

Good luck...

 

[jcwhite1288]

thats strange then that scarebird doesnt aknowledge any issue with using the 74 maverick MC, or why he even recommends that specific MC if it doesnt work.

 

[2+2GT]

What is the bore diameter of the S10 cylinder?

 

[TIGGER]

By worse pressure do you mean the pedal is too hard or takes a lot of force on the pedal to get the brakes to work properly? If so I have the same problem. I did my scarebird swap about 3 years ago (6cyl) and am not real happy with the amount of force required to stop the car but at least it stops and stops straight. I am used to it now but it would be nice to take a little less effort to stop my car.

 

[2+2GT]

OK, disc brakes 101. Disc brakes, by design, require significantly higher hydraulic pressure to operate. If you use a master cylinder with a smaller diameter, it will reduce pedal effort, but increase required pedal movement. This is why all disc brake Mustangs from 67-on had power boost. If low pedal effort was your goal, converting to disc brakes was a mistake, you should have put power boost on your drums. If the S10 master has smaller bore, use it.

And a blob of silicone at the bottom of the cup will retain the rod. Make sure the rod is all the way to the bottom while the silicone cures.

 

[PoppyMod]

Another thought is to get a line pressure kit from one of the brake vendors and check your pressures. You should have about 900-1100 PSI on the front and slightly less to the rear, so as, to not lockup under heavy braking. Obviously, you need a proportioning valve installed on the rear line to adjust. Do you have a 10 psi residual valve inline to the rear?
I'm running manual brakes after a number of years of power assist and, at first, thought it was too hard. But, have since, gotten used to them.
Good Luck!

 

[TIGGER]

OK, disc brakes 101. Disc brakes, by design, require significantly higher hydraulic pressure to operate. If you use a master cylinder with a smaller diameter, it will reduce pedal effort, but increase required pedal movement. This is why all disc brake Mustangs from 67-on had power boost. If low pedal effort was your goal, converting to disc brakes was a mistake, you should have put power boost on your drums. If the S10 master has smaller bore, use it.

And a blob of silicone at the bottom of the cup will retain the rod. Make sure the rod is all the way to the bottom while the silicone cures.

Thanks but my goal was to get rid of my insta-fade 9" drum brakes and get something that would stop straight and not fade out after one panic stop. The pedal effort was not what I expected that's all. As I said before I am used to it now as it has been four years since I did the swap. I used the Maverick MC as the instructions called out. I am just curious to know if others have the same pedal effort.

 

[phutch11]

I don't think that a blob of silicone is a safe recommendation for positive retention on the brake pushrod.

If that rod comes out for whatever reason, you've got zero brakes.

Just my two cents.

 

[Scarebird]

The reason we spec out the 74 Maverick unit is it has a good hydraulic match to the S10 calipers. This deal with the quick take up calipers is not a big thing; if you do the math you find it is worth maybe 1/4" of pedal swing. 90% of the time we find either inadequate bleeding or defective master (yes, even rebuilt units can be "shelfworn", ie. dried up seals).

We do indeed listen to our customers; quite a bit of feedback helps us improve the breed.

 

[Scarebird]

oh, and the silicone thing scares the hell out of us: the pedal rod may swing up to 10°, tearing up the bond. If you must use a S10 master, pull out the piston and bore a hole 1" deep for the rod to reside in.

 

[chockostang]

oh, and the silicone thing scares the hell out of us: the pedal rod may swing up to 10°, tearing up the bond. If you must use a S10 master, pull out the piston and bore a hole 1" deep for the rod to reside in.

Yes, No Doubt--Hell scared is not the word.

Dan @ Chockostang