I've always thought the brakes couldnt bleed to the floor unless there was a visible leak NOT due to just air in the lines. Air in the lines just usually gives you the spongy pedal. I can pump up the pedal and let it bleed to the floor 10 times and then get out and crawl under the truck and theres no sign of fluid anywhere, the reservoir doesnt even go down. Im at a complete loss here. The truck isnt at all driveable like this.

 

Sure sounds like a dead master cyl. Thats the only thing that can cause the fluid to "bypass" and let the pedal drop withou having any leaks. Does the truck have ABS? 4W, or just rear? If it has 4W ABS, there could be air trapped in the ABS Unit. Try going out and activating the ABS and then re-bleed the brakes. I have had to do that before. Also, can you "pump the pedal up". Meaning after the pedal falls.....can you pump it up and it hold?

 

There can be an internal leak that causes the pedal to bleed down.
Since the master was replaced the only thing left where this could happen would be in the proportioning valve.

 

Ok, so heres my next question. When i replaced the master cylinder I bench bled it with the plastic plugs that came with the master. They're the ones that just plug off the ports. Well the directions from the way I read them said to fill the resevoir and pump the piston until you can't move the piston in more than 1/8". Well when I did it the piston never stopped going all the way in, I could see that the bubbles stopped coming up out of the fluid but the piston never had any resistance. What EXACTLY should have happened?

The last master cylinder I replaced was quite a long time ago and that came with the bleeding kit with the short hoses you looped up into the resevoir. I've got no experience with the ones that just plug off the ports.

 

You should have built up pressure! If the master cyl was still going all the way in, no air bubbles , there's your trouble!

 

I went through all of this last year, i replaced EVERYTHING in the brake system , except for the vaccum booster , and the valve under the master cylinder, and i still had a pull in my brakes , and i ended up with the sinking pedal also , which i did not have before.

I changed the master out 4 times , finally ending up with a new , not rebuilt one , and still had problems, after 4 sets of front calipers , i got some that did not leave me with a pull in the brakes. I still had a sinking pedal though, which i don't know if i have ever solved. I talked to one of the best mechanics in my area about it , and he was rather stumped , but his suggestion was to use vise grips and clamp off all three flex lines tight enough that no fluid could pass , then step on the brake pedal ,and see if it still sinks, i did that , and my pedal was hard as a rock... I started releasing the vise grips one at a time ( i started at the back) and the pedal stayed hard until i released one line at the front , and the pedal started sinking again. releasing BOTH front flex lines and the pedal sunk just as fast as always. I have no leaks anywhere.

What i think the problem is , is the front flex lines ( which are/were new) are soft ,and they slowly expand/stretch under pressure, causing the pedal to drop slowly , with no leaks visible. Some day when I have the money , i am going to try some braided SS flex lines , and see if that makes a difference , they will not stretch or expand under pressure.

Sorry for the long post. Try clamping off your flex lines , and if the pedal still drops , the problem is somewhere in the system BEFORE the flex lines ( master cylinder). if the pedal is hard , release the clamps one at a time until the problem appears , this will help to narrow it down.

 

You think it should have built up so the piston couldnt move in at all with the supplied port block off plugs? I pumped it close to 30 times and it never pumped up hard.

 

It should have gotten considerably harder! Most of the time( that I can recently remember) they do get to the point that I can barely move the piston.

 

Im having the exact problem! step on the pedal and slowly sinks to the floor. but if i stand on it hard it dosent move... when i let up alittle it starts to bleed off. started one day when i was changing the front brake pads. i put a clamp on the cal to get the piston back in and it wouldnt move. had to open the bleed port to get the piston to move back so i could put on the new pads. makes me soo d@$& mad!

 

hey gearhd6, my breaks were so spongy they were almost not there at all. This weekend I replace the wheel bearing, not to fix the breaks, but because it was 100 % dead. Now the breaks are back to perfect...well for a dodge. I agree with the other posts, and I plan to replace break lines and bleed the system too. But i tell you, it sure made a difference..

 

Internal master cylinder leak was a possibility. Reman masters can still be bad especially aftermarket trash that is sold these days, but probably not. You have to eliminate other things before going back & replacing the master. Regardless, Start at the rear brakes & make sure they are both adjusted. Pull the drum off & make sure the automatic adjuster is engaged like it is supposed to. Mine was bent so I would adjust & it would soon back off. With everything installed correctly, pump the brakes to center the rears & adjust. Pump to center, re-check & adjust as necessary. You want some drag when you've finished adjusting. Flex lines can get weak & cause problems. I've only had this problem once on a car & went with braided stainless just because (Earls I think). The stainless leaked at the joints until they were cranked down. I think they should have sealed better & will try another brand next time at least on that car. The squeezing idea is a good one as it will eliminate 1 corner at a time. I split a hose & fit over vice grips so the line doesn't get cut. My pedal was getting soft & I adjusted today when I changed oil. Drums slid off easily prior to adjustment & I had to wiggle off after. Pedal is hard & ebrake works better. When brakes pull, it can be the caliper opposite the pull OR something interfering with the caliper causing it to hang. It's usually rusty or at least non-lubed pins causing the hang. Just make sure all the hardware is clean, greased & the caliper is sliding smoothly. Please report back when you find it.

 

I had the same problem, one of the soft lines off the front calipers was starting to bulge under pressure. Took some time to find it, replaced them and it made the brakes 100% better.
Cheers!

 

I started with repacing the rotors and pads on the front axle. the brake system was not opened. calipers were pushed back. Tested the brakes and pedal was spongy. changed master cyl. blead brks, no change. replace master again. no change. Clamped off front brk hoses pedal was hard. removed clamps pedal spongy. changed brk hoses, pedal still spongy. Clamped hose again, pedal hard. Replaced calipers, pedal still spongy. The brk system is on a 95 ram 2500 4X4 diesel. Any ideas would help. Thanks Paws

 

Make sure the calipers are on the right sides.
They will fit on either side but will be upside down.
With the bleed screw on the bottom of the caliper you'll never be able to get all the air out.

 

The bleed screws are on top.