soft brake pedal

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ir3333

ir3333

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two pumps and pedal is hi and firm but if i continue to push the pedal hard it very,very slowly goes down.
does this sound like a bad master cylinder or air in the master...i did bleed it before i installed it?
 
..tx '67!
it's new (rebuilt?) from Rock Auto.
i'll order a new one from Master Power Brakes...where i should have gone in the first place
 
If you don't have wet spots from fluid at any of the four corners it's the master cyl.
 
QUOTE=ir3333;1970450853]two pumps and pedal is hi and firm but if i continue to push the pedal hard it very,very slowly goes down.
does this sound like a bad master cylinder or air in the master...i did bleed it before i installed it?[/QUOTE]


Sounds like you still have air in the system. What you describe sounds like the pedal 'pumps up', meaning it gets firmer after more than one application. If that is the case, then it is definitely, 100% air. The fact that the pedal drops while keeping pressure on it kind of confirms that there is air somewhere. Can't compress a liquid, only gas or 'air'.

I never like when guys give a pat answer like 'bad master cylinder'. Sometimes it can take several tries to get all the air out. Which technique did you use to bench bleed the master and the rest of the system? Might need to do it a few times.

One thing that you might want to consider in regards to the master is whether it is matched to the rest of the components in your brake system, i.e., correct application and/or MC bore size in relation to wheel cylinder/caliper piston area.
 
brakes have been bled plenty with assistant on the pedal...no air anywhere now and no leaks.
i guess plugging the master cylinder outlets is the next step.
 
rmchrgr said:
QUOTE=ir3333;1970450853]two pumps and pedal is hi and firm but if i continue to push the pedal hard it very,very slowly goes down.
does this sound like a bad master cylinder or air in the master...i did bleed it before i installed it?
Click to expand...


Sounds like you still have air in the system. What you describe sounds like the pedal 'pumps up', meaning it gets firmer after more than one application. If that is the case, then it is definitely, 100% air. The fact that the pedal drops while keeping pressure on it kind of confirms that there is air somewhere. Can't compress a liquid, only gas or 'air'.

I never like when guys give a pat answer like 'bad master cylinder'. Sometimes it can take several tries to get all the air out. Which technique did you use to bench bleed the master and the rest of the system? Might need to do it a few times.

One thing that you might want to consider in regards to the master is whether it is matched to the rest of the components in your brake system, i.e., correct application and/or MC bore size in relation to wheel cylinder/caliper piston area.[/QUOTE]

Air compresses but returns the pedal.
A pedal that continues to fall with pressure is a fluid leak, be it at the brakes themselves or the master cylinder internally.
So if it does truly keep falling and none of the corners or around the master are wet, it's the master cyl.
 
it falls very,very slowly...barely detectable.
..My thought was that fluid is passing one of the master cylinder pistons ever so slightly.
 
ir3333 said:
brakes have been bled plenty with assistant on the pedal...no air anywhere now and no leaks.
i guess plugging the master cylinder outlets is the next step.
Click to expand...

That would be a good thing to do. You can also try to isolate which circuit the problem may be by plugging the individual ports.

I just went through a similar situation with my own car - all new brakes, bled properly, no leaks but no pedal. Ended up having two separate issues; 1) I was using the old drum brake ditribution block which does not have the hold off feature that the disc brake one has and 2) the master cylinder I chose had too large of a bore for the pistons in my front calipers which resulted in inadequate pressure. Couldn't stop the car. Got the correct/appropriate parts and all is right with the world. Just something to consider if you have any non-factory parts.

Before I figured all that out though, I kept bleeding over and over always with the same result. I must have run about 2 gallons of brake fluid through the system thinking there was air in there somewhere but there wasn't any.
 
i do have a small master and large volume calipers but the pedal should not fall.
what master did you end up using 1" or 1 1/32" ?
i have the 2 3/4 piston calipers in my duster with a 1" master and it is like power brakes.
..this system is the 4 piston Kelsey hayes which are larger volume and i am using 15/16'
 
ir3333 said:
it falls very,very slowly...barely detectable.
..My thought was that fluid is passing one of the master cylinder pistons ever so slightly.
Click to expand...

Sounds about right, and is what it would lead me to believe also.
 
ir3333 said:
i do have a small master and large volume calipers but the pedal should not fall.
what master did you end up using 1" or 1 1/32" ?
i have the 2 3/4 piston calipers in my duster with a 1" master and it is like power brakes.
..this system is the 4 piston Kelsey hayes which are larger volume and i am using 15/16'
Click to expand...

Not sure what you mean by falling, but I'll infer you have that sensation because the smaller bore requires more travel and pedal effort before it starts to grab. That's the tradeoff for more pressure at the pistons. Not sure if that is accurate for your situation.

I have SSBC aluminum KH replacement calipers on my car. The pistons are a tiny 45mm. I now have a 15/16 MC which seems to work. Initially I had a 1 1/8 master, obviously way to big but I didn't realize the effect bore size had on line pressure until I had no brakes. I could push the pedal to the floor and the car wouldn't stop. It was confusing too because the pedal was firm in the garage but apparently was not generating enough pressure at the pistons to stop the car from a slow roll. That's really what it felt like too if that makes sense.

I bled the brakes on my car over and over with no change so I figured there had to be a mismatch somewhere. Both MCs I have are brand new so the chances of both being bad out of the box were pretty slim. Started reading up on the subject a little deeper which led me to the bore size idea. The KH pistons are on the small size so you need the smaller bore master to generate adequate pressure. The '73-up calipers are single piston but actually have a larger area than the KH calipers so you can get away with a 1 1/32 or 1 1/16. 1" would probably be too small for the larger piston, the pedal would travel too far before it started working. Might be your issue right there but don't quote me on that.

Also, the disc brake metering valve is what you need if you don't have one. A drum brake valve is just a distribution block, the pressure is the same on either side. The disc brake valve keeps the pressure to the rears lower when you press the pedal so they don't lock up before the fronts start working. Incidentally, if you have a disc brake valve and then installed another prop valve after that, its going to decrease the pressure to the rears even further, even if the valve is backed off all the way.

Not sure if any of this is redundant or relevant to you but just thought I'd throw it out there.
 
actually 4 1 5/8 pistons in a kelsey caliper are 8 sq inches where the larger single piston caliper at 2 3/4 is only 6 sq in.I have a 1" master in the Duster with the 2 3/4 piston caliper and there is a bit of travel for a rock hard pedal, but for normal driving the brakes stop quickly long before the pedal ever gets hard....very nice.
i do think i need at least 1" master for these kelsey discs and may be why i am pumping twice to raise the pedal initially...and i have since found some seepage ..
 
ir3333 said:
actually 4 1 5/8 pistons in a kelsey caliper are 8 sq inches where the larger single piston caliper at 2 3/4 is only 6 sq in.I have a 1" master in the Duster with the 2 3/4 piston caliper and there is a bit of travel for a rock hard pedal, but for normal driving the brakes stop quickly long before the pedal ever gets hard....very nice.
i do think i need at least 1" master for these kelsey discs and may be why i am pumping twice to raise the pedal initially...and i have since found some seepage ..
Click to expand...

When you measure caliper piston size, for whatever reason, you only measure one side on a four piston caliper so it's really only around 3". Using the SSBC calipers I have on my car as an example, piston area comes out to a whopping 2.94 square inches. Awesome.

15/16 master cylinder is the right size for the KH brakes. You'll have to live with a little longer pedal stroke but the pressure will be correct for the application.
 
that doesn't make sense but may be correct
Are your ssbc 4 piston calipers floating,the kelsey hayes are fixed so all 4 pistons move the same amount
 
ir3333 said:
that doesn't make sense but may be correct
Are your ssbc 4 piston calipers floating,the kelsey hayes are fixed so all 4 pistons move the same amount
Click to expand...

Fixed. They are copies of the originals, just in aluminum. Again, not sure why the piston area is calculated that way. Read about it here.

The only other thing I would suggest before you start throwing parts at the problem is to verify how much pressure you have at the wheels. There is an inexpensive pressure gauge kit to do this. Click here.
 
yes he calculates one side of the caliper but later he doubles that calculation for brake torque and volume.
..excellent thread!
 
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