Soft Brake Pedal

[tnoftsger]

Alright brake experts, can't seem to get a firm pedal.

Napa 36300 master cylinder, 3/4" bore, approximately 1" of stroke before it bottoms out
5.5:1 pedal ratio, no binding and getting full stroke
stock 1966 mustang drum brakes 10" X 1.75" shoes
GM intermediate disk brakes on front 2.934" piston

Steel line run from master cylinder to front of frame, stainless steel braided hose to calipers.
Steel line run from master cylinder to rear kick-up, stainless steel braided hose to tee on rear end.

bench bled the master cylinder with plugs first and then with lines run from the outlet back into the resevoirs. Pump and then wait 15-30 seconds, pump again wait, etc.

bled the lines by pumping and hold, release bleeder futherest from the master, then move to opposite wheel.

with the one port plugged and rear brakes connected, i get a firm pedal, connect the disk brake system and get soft spongy pedal.

Switch the lines on the ports on the master cylinder, no help.

The engineer in me had to do the math:

3/4" master bore times the 1" stoke you get (pi*r^2*stoke) nets 0.44 cubic inches of volume.
the 0.44 cubic inches of volume translated to the 2.934 inch caliper piston (0.44/pi*r^2) nets 0.065 inches of movement, divided by the 2 caliper is 0.03 inches of piston travel.

I know that the pads ride very close to the rotor, but it seems that i don't have enough volume per pump of the pedal to compress the pads solidly before i run out of stoke.

whatta ya think?

Thanks in advance

Tom

 

[austinb]

You will need 2 psi residual valve in line with disc breaks and 10 psi residual valve inline with drum breaks. And a proportioning valve to match the rear and front breaks so one won't lock up before the other. The residual valve is a directional valve that will keep all the fluid from returning to the master cylinder causing soft pedal. I had the same problem cause I didn't think I would need them.

 

[bucketkids]

Do you have a residual valve inline that hold 2PSI presure and keeps cliper from opening all the way back up???

 

[RPM]

Before you buy all those fancy valves and stuff. Try driving the car and seat the pads and shoes. I don't have any of that stuff on my T.

 

[Old Rotor Flap]

Pi R Square....

No, Pi R Round, Cornbread R Square

 

[akitagandy]

:rofl: :rofl: :rofl: :rofl: :rofl:

 

[Rick]

Before you buy all those fancy valves and stuff. Try driving the car and seat the pads and shoes. I don't have any of that stuff on my T.

I dont have the residual valves i think the problem is the piston size i have a 1" piston and have maybe 1/2"travel.Again Just my opinion.I also have 3/16 brake lines.

 

[GT63]

Alright brake experts, can't seem to get a firm pedal.

Napa 36300 master cylinder, 3/4" bore, approximately 1" of stroke before it bottoms out
5.5:1 pedal ratio, no binding and getting full stroke
stock 1966 mustang drum brakes 10" X 1.75" shoes
GM intermediate disk brakes on front 2.934" piston

Steel line run from master cylinder to front of frame, stainless steel braided hose to calipers.
Steel line run from master cylinder to rear kick-up, stainless steel braided hose to tee on rear end.

bench bled the master cylinder with plugs first and then with lines run from the outlet back into the resevoirs. Pump and then wait 15-30 seconds, pump again wait, etc.

bled the lines by pumping and hold, release bleeder futherest from the master, then move to opposite wheel.

with the one port plugged and rear brakes connected, i get a firm pedal, connect the disk brake system and get soft spongy pedal.

Switch the lines on the ports on the master cylinder, no help.

The engineer in me had to do the math:

3/4" master bore times the 1" stoke you get (pi*r^2*stoke) nets 0.44 cubic inches of volume.
the 0.44 cubic inches of volume translated to the 2.934 inch caliper piston (0.44/pi*r^2) nets 0.065 inches of movement, divided by the 2 caliper is 0.03 inches of piston travel.

I know that the pads ride very close to the rotor, but it seems that i don't have enough volume per pump of the pedal to compress the pads solidly before i run out of stoke.

whatta ya think?

Thanks in advance

Tom

I have the same problem and I have residual valves in the lines. In the next few weeks I'm going to re-do all of the brakes lines and remove the residual valves. The local brake shop said I didn't need them, because it's a closed system and if it's bled right the fluid won't drain back.

 

[rooster57]

HYD Brakes are a pain i think a stick in the spokes or just dont go anywhere where brakes are needed. :rofl:

 

[HAceT]

I had the same problem last year and now it has started again after a year of driving. I'm back to a very very soft pedal and no brakes with out pumping several times. I'm going to replace the MC to see if that helps.

 

[TexasT]

I had same issue with mine and I had residual valves come to find out one of Speedy Bills valves was stuck open, replaced with a wilwood brand and worked every since

 

[tnoftsger]

I have thought of the residual valves, and that it might help to keep the pads closer to the rotor and therefor take less volume to move the pads closer.

I also failed to mention that pumping the pedal repeatedly and quickly builds a little more pedal, but not much. Man i wish i had a pressure gauge to put in the line somewhere to see what kind of pressure i'm getting.

Waiting on the new drive shaft. Old one didn't fit after i stretched the whole car 10", lol.

I don't really subscribe to the idea that the fluid will drain back to the master cylinder. If it did then i wouldn't have needed to top off the resevoir tonight after all the bleeding i did yesterday, it would have just "drained back" and filled it. However i can see where a residual valve could keep the piston from receding from the rotor some and may help.

I might run out the the garage and put a c-clamp on both of the inboard pad and caliper and see if the pedal firms up when the piston basically can't move. If it is a volume problem the pedal should be firm, RIGHT?

 

[409T]

I have the same problem because my calipers are rotated so that the bleeder is not at the top of the reservoir and bleeding them in this orientation will leave a bubble of air above the bleeder. I have to take the calipers out of their brackets and hold them so that the bleeder is at the very top, and then I can get all the air out.

 

[putz]

This is the master i recommend Napa 36440 more volume.Cheaper also 73 compared to 124. 15/16 bore compared to 3/4 bore.

 

[WEDRVIT]

Alright brake experts, can't seem to get a firm pedal.

Napa 36300 master cylinder, 3/4" bore, approximately 1" of stroke before it bottoms out
5.5:1 pedal ratio, no binding and getting full stroke
stock 1966 mustang drum brakes 10" X 1.75" shoes
GM intermediate disk brakes on front 2.934" piston

Steel line run from master cylinder to front of frame, stainless steel braided hose to calipers.
Steel line run from master cylinder to rear kick-up, stainless steel braided hose to tee on rear end.

bench bled the master cylinder with plugs first and then with lines run from the outlet back into the resevoirs. Pump and then wait 15-30 seconds, pump again wait, etc.

bled the lines by pumping and hold, release bleeder futherest from the master, then move to opposite wheel.

with the one port plugged and rear brakes connected, i get a firm pedal, connect the disk brake system and get soft spongy pedal.

Switch the lines on the ports on the master cylinder, no help.

The engineer in me had to do the math:

3/4" master bore times the 1" stoke you get (pi*r^2*stoke) nets 0.44 cubic inches of volume.
the 0.44 cubic inches of volume translated to the 2.934 inch caliper piston (0.44/pi*r^2) nets 0.065 inches of movement, divided by the 2 caliper is 0.03 inches of piston travel.

I know that the pads ride very close to the rotor, but it seems that i don't have enough volume per pump of the pedal to compress the pads solidly before i run out of stoke.

whatta ya think?

Thanks in advance

Tom

You did the math (very good) you've got the answer, now just fix it. Larger master or smaller calipers? If the caliper seals are good and the pins are perfect they will normally retrack about .010 - .015 but if the seals are stiff and the pins stick they may retrack well over .020. Oh and don't forget the alignment of the bracket to the rotor if it's not perfect the pads won't square on the rotor. I use a 3/4 master with a pair of Harley four pistons fixed calipers up front and four 86 Vette single floaters in the rear. The Harley calipers measure out to 1.9 ea. and the vettes are at 1.6 ea. but they only retract about .010. A 10 lb. line check valve on the rear helps to keep gravity from draining a small amount (about a half pedal) back to the master on long road trips where you don't use the brakes very often, oh by the way they don't hold 10 lbs. You're going to need a hellva c-clamp if you going to try and hold that piston to create a solid pedal. Good Luck Harley

 

[tnoftsger]

i didn't get a chance to try out the c-clamp idea...The wife had other things she thought i should be spending my time on.

I might try a residual valve. Cheapest option.

The bleeders are on top of the calipers so i don't think that is an issue.

Again the math:

15/16 bore MC will cause the caliper pistons to move 0.05"
Vega calipers have a piston that is 1.872 dia which translates to 0.08" with the current 3/4" bore MC
Switching both calipers and MC nets 0.125"

So, i have options i guess.

 

[WEDRVIT]

i didn't get a chance to try out the c-clamp idea...The wife had other things she thought i should be spending my time on.

I might try a residual valve. Cheapest option.

The bleeders are on top of the calipers so i don't think that is an issue.

Again the math:

15/16 bore MC will cause the caliper pistons to move 0.05"
Vega calipers have a piston that is 1.872 dia which translates to 0.08" with the current 3/4" bore MC
Switching both calipers and MC nets 0.125"

So, i have options i guess.

Just for your own veiwing pleasure put a dial indicator on the piston and check the amount of retraction, could be interesting.
GM intermediates are over kill up front anyway. Harley

 

[HAceT]

This is the master i recommend Napa 36440 more volume.Cheaper also 73 compared to 124. 15/16 bore compared to 3/4 bore.

I wanted to say Thank you Putz for that part number and information, man I love this site. I took the cheaper route I guess......I took the NAPA number and crossed it with the Advance Auto Parts and got 101614 and for only $15.99. I installed it today and have the best pedal I've ever had in the car. Again Thank You. I am now happy with my brakes and can ride again.

 

[putz]

Not a problem anytime.

 

[HAceT]

well I put 654 miles on the new feeling brakes and stiil have a AWESOME pedal.........Man I can't THANK Putz enough or this site.