Soft Pedal

[coupe351w]

:chin:Sorry, I know I have asked this question here before. Just got the car down off the jack stands and took it for a spin. Runs good but still that darn really soft brake pedal. Here is what I have done in the past. New master cylinder, rebuilt booster, Both rebuilt front calipers. stainless flex lines to the front calipers, I have put rear disc system on the back and I have bled the brakes so many times that I probably went through 5 gallons of brake fluid. Forgot I have tried at least three different master cylinders, one was a rebuilt 85 SVO mustang ( larger bore) . Same thing soft, very soft brake pedal. The car will stop, but with that pedal you wonder how. Any ideas you guys have will be helpful.

Thanks Dean.

 

[73pony]

That is really strange. I have completely stock brake set up from power disc and rear drums and my pedal is rock solid. Still the original master cylinder and calipers. Could there be a weak brake line somewhere or a fitting leaking? How did you bleed the brakes? Did you use a vac pump or the old fashion way?

 

[coupe351w]

No leaking, I use Vac pump to bleed.

 

[rocket366]

The master must be "bench bled" if you dont know what that is youtube it.

I was amazed how much air was in my lines even after normal should be good bleeding

when I did a flush starting with furthest away and watching what came through a length

of clear tubing in a clear plastic bottle with the end of the tube submerged so no air

was sucked back in. I mean AMAZED!

Paul

 

[c9zx]

No leaking, I use Vac pump to bleed.

Have you tried the "old school" 2 man method. It is ann old school car and that method always works for me, on older cars. Please don't take offense but, could the front calipers be reversed left to right? If so, the bleeder screw is not at the highest point and therefore will not bleed properly. Let us know what you find. Chuck

 

[Bentworker]

What kind of combination / proportion valve are you running?

 

[midlife]

How old are any of your rubber brake lines? If old, they can balloon out with pressure, giving you a weak pedal. You'll never see this problem while bleeding brakes.

 

[Don C]

I've never cared for vacuum bleeding, difficult to control air infiltration around the bleeder valves, plus it's too easy to suck the reservoir dry. I prefer either the two-person method or pressure bleeding.

It's fairly easy and inexpensive to make your own. This is an example:

http://faculty.ccp.edu/faculty/dreed/Campingart/jettatech/bleeder/index.htm

 

[coupe351w]

To answer all, I did bench bleed all the master cylinders, made sure calibers are on right with bleeders at highest point, stainless braided lines up front new and new rubber line in back, have tried old two man bleeding also, only thing I have not changed is it still has the old proportioning value. Been meaning to change it, do you thing that could be the reason for the soft pedal?

Sent from my SM-G925V using Tapatalk

 

[coupe351w]

Thanks Guys also.

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[Bentworker]

I don't think the proportion ratio of the stock combo valve is correct for rear disks. I don't think that would cause a spongy pedal though. Personally I just dislike combo valves, that evil little shuttle valve inside seems like a pain. I finally tossed the one on my 82'GMC and put in an adjustable Wilwood proportion valve in it's place.

I can't come up with any good reasons why you have a soft pedal other than air trapped somewhere, or something weird. Something weird could include excessive rear axle endplay, pushing the piston in the rear calipers as you drive, then the piston has to travel further causing excessive pedal travel. Might be worth checking if you run out of other ideas.

 

[Don C]

You didn't say whether or not you replaced the flex line at the rear axle. Midlife is correct about old rubber lines.

 

[eddyw]

Same problem here in the beginning (soft brake pedal).

Do you adjust the stem in the booster(that little nut on the stem 1 or 2 turns out)

see pic.

 

[coupe351w]

Don C, I did replace the flex line at the rear axle and eddyw I also adjusted the stem on the booster. I checked it and re checked it. I guess air could still be in the lines. Will try and bleed them again and get an adjustable proportion valve. This is Driving me Crazy

 

[Bentworker]

This is why I tossed out the unlikely but possible end play idea...

I have disk brakes on the back of my grande. The vendors brackets were way out of specification and bearing recess on the caliper bracket was bored too deep (small bearing 9" by Mustang Steve). I ended up using VW flywheel shims to get it back to where it needed to be. I know it is a long shot, but I figured I better explain why I mentioned it just in case.

 

[coupe351w]

Thanks Bentworker, I will have to look into that. The rear set-up I have was from Summit Racing.

 

[c9zx]

Most conversion kits require proper adjustment of the parking brake and some require loosening the caliper and rotating the bleeder to true vertical with the pads still contacting the rotor so the piston does pop out when the brakes are applied while bleeding (sounds like a 3 person job). Did any parking brake adjustment instructions come with the kit? Chuck

 

[Jeff73Mach1]

You also mentioned a larger bore MC

For greater line pressure you need a smaller bore. SSBC has an inexpensive line pressure testing gauge-it was helpful in setting up my brake system when I was having similar problems.

My reading up on it leads me to believe a 1" bore MC is probably ideal for 4 wheel disc brakes with single pot calipers.

Finally, using ceramic pads, especially race formulations will make for poor stopping power as they need more heat that you consistently can generate. This is probably more true on the front unless calipers were upgraded. Changing my front pads back to semimetallics was a big improvement

 

[c9zx]

This video describes one method of adjusting the rear parking brake. Your brakes may be different. http://www.getdiscbrakes.com/support/videos

If you are using a disc/drum master cylinder be sure to remove the residual valve in the rear circuit of the master cylinder. Chuck