spongy brake pedal has me stumped

i've searched everywhere on the jalopyjournal, and other forums as well, and i'm at a loss.

i have a disc/drum setup on my f1 (with a malibu front stub). late this fall i had noticed some leaking around the master cylinder, so much in fact that it was starting to bubble up the paint on my inner fenders.

so i kept adding to my MC until i could get to it. i noticed that only the front reservoir would get low, and sure as hell one day it got too low. So i had run the front half of the MC dry but was replacing the calipers anyway so i did that, then bench-bled (in the truck) the front reservoir just to get the air out but did not see any.

so here's where i'm at; with new calipers and brake hoses, i've tried gravity bleeding the system, tried the two person method, it seems like no matter what, i need to pump the brakes twice to get them to stop. it never takes more then two but almost always does.

also, this may be related or not, but my MC cap is leaking (which is where all the fluid on my inner fenders was coming from), but nothing else is. all the lines are fine but the cap seems to be leaking almost profusely.

this really baffles me since the brakes were working great all summer. the only difference between then and now would be that i've switched camshafts to a much more mellow grind, so it should supply more vacume but i'm not even sure that would have anything to do with my situation.

any help would be great!

 

Sounds like you still have air in the line .... start from the farthest wheel away and work your way in

 

Did you change the master cylinder? It could have an internal leak.

 

i had replaced it around this time last year when i was initially getting it on the road. you think it could be bad again?

 

On my m/c the front part of the reservoir (closest to the radiator) is for the rear brakes.

 

ya im thinking that might be the case. no big deal, $15

 

i was surprised to see, on a new master cylinder, a warning that said do not push the master cylinder all the way in when bleeding. i guessed it was for that exact reason.

 

hmm, so i got my master cylinder exchanged. bench bled it in a vise, threw it in the truck, "bench bled" it in there a little for good measure. Bled all the lines front and rear, no leaks...now its even worse then before! the pedal wont even get stiff at all now.

i had my girlfriend in there working the brakes, i did the method where you open the bleeder, push and hold down the pedal, close the bleeder then let up on the pedal.

is there a better technique? tomorrow im going to try bleeding them with my buddie just in case.

this is going on two weeks ive been dicking with this, im starting to get frustrated seeing as how this is the only thing between me and taking this bitch to LSRU

 

pump 3 times, hold it down, open bleeder, close bleeder, release pedal, pump 3 times, hold it down, open bleeder, close bleeder, release pedal,
pump 3 times, hold it down, open bleeder, close bleeder, release pedal, pump 3 times, hold it down, open bleeder, close bleeder, release pedal,

move to other end,,,,,,


pump 3 times, hold it down, open bleeder, close bleeder, release pedal, pump 3 times, hold it down, open bleeder, close bleeder, release pedal,
pump 3 times, hold it down, open bleeder, close bleeder, release pedal, pump 3 times, hold it down, open bleeder, close bleeder, release pedal,

 

Carbed, I have had the same problem as you and Oldsteelsteve. It turns out as well that it was the MC. Try swappin it out and that may fix ya.

 

moving to that technique tomorrow. thank you

 

when your holding the pedal down after you have pumped it 3 times use a good amount of force, you want the fluid to come shooting out, this helps to move any air along that might be trapped in a high spot, also make sure your rear brakes are set up.

 

Check the location of the bleeder screws in the calipers. We had this on a P/u with A/M brakes. The bleeder was below the top of the line. No way would it bleed properly. We had to jack the front of the truck up to get the bleeder above the line height. Then it finally bleed the air out. That took some head scratching!!!

The Old Tinbasher

 

Does your brake system have residual pressure valves installed? They require 10 psi for drum brakes and 2 psi for disc brakes. This will hold fluid in the calipers and wheel cylinders to keep them from bleeding off while not in use. This will cause you to have to pump your brakes to get enough fluid into the brake componants to get them to proper clamping pressure. Just my 2 cents worth

Rod

 

my MC is firewall mounted therefore much higher then my calipers/drums so i was under the impression residual valves weren't needed.

tinbasher what exactly do you mean? my bleeder screws are on the lower side of the caliper (around 10 o' clock on my rotor) but definately below my MC.

 

the bleed screw needs to be at the top to remove all the air, your brakes were working ok before all this started?

 

carbed ... your getting good advice but personally I think you just dont have the bleeding process down .... like I said earlier start at the wheel cylinder farthest from the MC ant work towards it .... and dont let the pedal return up with a bleeder still open and dont let the resevoir get too low while bleeding .

The chances of having two master cylinders going south on you is just not probable ... I doubt this is your problem.

 

thanks for all your help guys, really!

so as oldsteelsteeve suggested, heres a few more specifics on my system:

chevelle/malibu front stub with factory style disc brakes (i.e. stock caliper position and calipers)
the MC has two lines going down to a distribution block mounted on the frame-rail with two going out to each front wheel, then a single line going towards the rear. about where the bed starts is one of those $45 wilwood proportioning valves, comes out of there and goes to a factory chevelle rubber transition line onto the rear axle.

as stated earlier, im going to have my buddie help me tomorrow after work so i should know by then if it was just a matter of bleeding it wrong.

 

Power bleed because really fine air bubbles are a bitch to bleed out. $.02

 

Buy some "Russel" speed bleeders!

Circuit racers used them all the time, it becomes a 1 man job and fast

 

I think it may have been bled wrong. There's some finesse to bleeding brakes and all it takes is one mis-cue that sucks air into the system which puts you back to square one. Also adjust up the rear drum brakes, that will give you a better pedal overall.

Bob

 

Please don't overlook pedal ratio also. I had the same problem. I had a 6 to 1 ratio peadal with power brakes and I modified it to 4 to 1 and it fixed my problem. I thought it was air in my lines also.

 

I think the first thing anybody should do that's seeking help for a problem on here is describe their set-up in it's entirety...........in this case the brake system. Saves bandwidth.

 

Generic response to any and all spelling Nazis, "Talk to my hand!"


What does 'talk to my hand' mean?

 

Are you dead sure you've 'bench bled' the m/c properly?

You mentioned you'd done it on the vehicle. Whilst this is technically possible given that most m/c's seem to point upwards (from the pushrod) I have my doubts about properly 'bench bleeding' it in situ (the m/c needs to point down to get the air to come up and out).

Then you did it on the bench - great - but was it pointing down when you did that?

Hope this isn't sucking eggs stuff - but you've not mentioned it and there's always that chance!

Chris

 

I believe this is it.

I have a ponycar, and after I added front disc, it started doing this. I can get about a month out of a bleed, and then I lose rear brakes. Not completely lose, but they only come in after a couple of pumps. The MC leaks like crazy, and has been spraying the underside of the hood. It will run the chamber for the rear (the front of the MC) dry after about a month. I keep it filled up, and it minimizes the issue.

There's supposed to be a residual valve in the MC, but most rebuilders remove it, and don't reinstall it from what i've read. It's a serious saftey issue IMO, but it's supposed to be the end user's responsibility I guess. There's supposed to be a way to check for it, but I don't know how. That's why the MC is leaking.. there's a back pressure now because the residual valve isn't there to stop it. I bought a residual valve, but haven't installed it.

 

check the adjustment on the rear shoes. Adjust them out pretty snug against the drum and check the pedal then. Just an idea and free to try.

 

I was fighting an issue with my F100, I replaced everything and put a pedal assembly out of an auto truck since the pedal was bigger and then my brakes went to shit. Always felt spongy, I had to adjust the pedal rod length and now they are good.

 

The absence of residual(s) in mid-'70s and earlier drum systems can allow air to enter past the wheel cylinder cups during fast brake release, which will result in a gradual loss of braking, along with increased pedal travel, and an extra pedal pump or two to regain a decent pedal and braking. You are correct that some new or rebuilt masters that had residuals in the original application do not have them now, requiring an inspection of the master cylinder that is to be used.

The only time there is any "back pressure" is during brake release, when fluid returns to the master, but is very low in value, and cannot cause any leaks in a normal air-tight system.

The master cylinder, as well as the entire brake system, should not have any leaks, period! You've got to find out what is causing the leaks, and correct the problem(s). Bad or loose fittings and poor flares are two common causes for leaks.

Checking for internal residuals on a dual master is easy. Use a paper clip or small drill bit and GENTLY push it into the master outlet ports. If you feel resistance, there is a residual present, otherwise the object will easily enter into the bore.

The picture below shows typical internal residuals located behind the tube seats. (This picture is of a late '60s drum/drum master).

 

well Mr spongy brakes how goes the battle?