47-55.1 low brke pedal

[klutz51]

Probably heard it 100 times but , under cab power master with disc/disc, have the inline valves, bleed the system over and over and no air, but the pedal acts like air in the system. pedal pumps up , let off and goes low but pumps up. I have used both vacuum and pressure bleeders, no air . Could I have air trapped somewhere I cant get?

[mr48chev]

It could be but there are some later model "low drag" calipers that are designed so the rubber pieces pull the pads back off the rotor to reduce drag that use what is known as a Quick take up master cylinder to push the pads back against the rotor without a lot of pedal travel and when you run a master cylinder designed for the older calipers (say 73 Camaro" you run into that issue.

[nvrdone]

what in line valves? if they are the 10# residual pressure valves I understand those are only for drum brakes. some one correct me if im wrong.

[klutz51]

I have 2 lb valves on both front lines and one on the rear. It is 4 wheel disc
.

[geezer#99]

What’s your pedal ratio?
What’s your m/c bore size?
How much vacuum does your motor produce at idle?

[klutz51]

The kit is the cpp disc disc kit. I dont have any vacuum to it yet but when i bleed the system the pedal pumps up perfect then when you let off for a few seconds it goes almost to the floor like air in the system.

[geezer#99]

Where are the 2# check valves located?
Do you have new brake hoses too?

[klutz51]

As close to the proprtion valve i could get them. One on each front line and one on the rear.

[klutz51]

All the brake lines are new

[geezer#99]

Is the prop valve centered?

And though I asked before, what’s the pedal ratio?

Is the booster hooked up?
Check valve in place?

[klutz51]

Not sure but when the pedal is pumped up it is in the perfect position

[geezer#99]

Info on pedal ratio.
https://techtalk.mpbrakes.com/how-to...ng-pedal-ratio

[dsraven]

-check the master cyl bore travel for a full stroke and compare to what you get at the pedal for push rod travel if there is no master cyl connected. in other words, make sure you have enough pedal travel to match the needs of the master cyl.
-check what the calipers need for fluid displacement and ensure your master cyl is matched to that
-like mr48 says, see what you have for calipers, low drag etc.
-if the pedal is hard and doesn't drop once it gets pumped up then you need to see why it takes that much fluid to get the pads out against the rotors. what do you have for rear calipers? if they are the ones with a built in park brake in the calipers piston, like older eldorados have, the park brake lever on the caliper needs to be adjusted. if not adjusted proper it takes fluid to get the piston out to the rotor, thus the low pedal.
is it like this?

https://www.youtube.com/watch?v=opjdu8DdVeA

[mr48chev]

I'm thinking that you have it set up as close to right as you can get it.
I had my wife'd mini van do something similar a few years ago after I changed a steel brake line and found out that one of the flares on the line wasn't sealing perfectly and was letting air back into the system but not showing a fluid leak. We would bleed the brakes she would drive it to work for about two days and we had to bleed the brakes again. I pulled the line off my little Ram panel van and swapped them and that fixed it. Especially if it seems to be happening at one end you might consider checking the flare fittings.


The only other thing I can think of is that it isn't that hard or expensive to make a pressure bleeder using a one gallon weed sprayer

This one is pretty simple http://faculty.ccp.edu/faculty/dreed...eder/index.htm

You could make a plate for the top of the Mc with a piece of steel or aluminum plate that was large enough to cover the top of the master cylinder and tap it for a fitting to hook to the hose and make a gasket out of a piece of rubber. You would have to figure out how to clamp it on the master cylinder but maybe a modified cheap C clamp would work there. That would let you push any and all air out of the system.

[dsraven]

If the brake pedal gets hard after pump up and is not spongy then it is likely a problem with fluid displacement. If you had drum rears I would say they need adjustment. Since you have disc disc look for a reason the pedal needs to be pumped up to get a hard pedal. If the master is on the frame check ALL the linkage for ANY slop in connections that would cause the master cyl to not get a full stroke with a full pedal stroke. Ensure the pedal is returning fully and that coincides with the master returning fully. I dont just mean the linkage here, I mean the actual master cyl plunger. Next check the booster operation. This is all after you do a full, hard pedal, brake application and then check each caliper to see if it retracts properly but not too far. Like mr48 says, possibly you have those free wheeling calipers and a master cyl that is marginal in bore size so it doesnt displace enough fluid required by 4 of those calipers.
Some more system information would be good.

[Jemezcrusher]

I have four wheel discs and the brakes do soften up .

I was recommended to bot a resid 20 on the front brake line and 80 (maybe 70?) on the back line and that would take care of it

[dsraven]

I recommend, before spending money, to go through the entire mechanical linkage on the pedal system. ensure the connections and pivots are all tight with minimal movement. a bit of movement in each connection could mean a substantial amount at the actual master cylinder. check the adjustment of the push rod between the booster and the master cylinder as well to ensure it is within spec. it has to have some gap but too much gap can mean more pedal travel before the master cylinder gets pushed. not enough gap could mean the fluid doesn't have a chance to get back into the reservoir or the reservoir can't fill the cylinder bore. then go down the line of hydraulics from there. ensure each connection is tight and there is no damp spots. a high spot in a steel or rubber line can also hold an air pocket, like a pee trap in a sink drain holds water because it is a low spot. a pressure bleeder would be a good option after completing the checks because it provides a constant pressure instead of a pulse and that can help flush out air from any high spots. bleed the furthest wheel first, ensuring the fluid coming out is clear and no bubbles etc, a piece of clear tubing with a tight fit on the bleeder screw will help you see better instead of an open bleeder screw. another thing that can happen when bleeding the old fashioned way, without a pressure bleeder, is the bleeder screws can leak air back into the system from around their threads. I usually just open the bleeder a 1/2 or 1 turn to help that not happen.
anyway, if the pedal is spongy with the engine off, so no power assist, then you still have air in the system or you have a caliper that is not able to move properly so things are "springing" under the force of the brake system. also, what style of calipers do you have on the rear? I would say the residual valves at 70 or 80 would be holding too much pressure in the system. typically a couple of pounds of residual valve would be all that's needed for a low mount master cylinder with disc brakes. if you had drums out back then a 10 psi res valve is typical. another thing to check could be the master cylinder to see if it has residual valves in it. some rebuilt ones are labelled disc/disc but they used to be disc/drum. in that case you can possibly have an old residual valve in the port for the rear brakes if the rebuilder forgot to remove it. google it for more info.
here is a link from speedway describing a typical hotrod brake system with a low mount master cylinder.

https://static.speedwaymotors.com/im...ake_System.jpg

another blurb on systems in general

https://www.hotrod.com/articles/sele...bing-pointers/

again, some pics would really help us to see what you are working on and sometimes some fresh eye on a problem can see something you habitually overlook because it is "good" in your mind from doing the build.

[MiraclePieCo]

More than once I have solved stubborn bleeding issues with speed bleeders.