Brake pedal drops to the floor

[jfrancom101]

So I have my 1985 gmc 2500. Originally it was a diesel but currently has a 5.7 that was dropped in there.
When I try to press the brakes the pedal gives no resistince and drops to the floor except maybe the lst 1/2 inch where it applies proably 10% brakes. obviously not safe to drive. I first tried blleding the lines and broke the bleeders so I got new cailpers and shoes on there. after bleeding the brakes still drop like before. I assumed it was my MC and replaces it after bench bleeding it. I'm having the same problem. What is could it be? It has the hydrobooster, but I'm less worried about having power brakes as I am having brakes at all that will stop the truck. Any thoughts?

 

[SquareRoot]

Your from Idaho. I'm from Idaho. You have an 85 k20. I have an 85 K20. You have hydroboost. I have hydroboost. Now that we have established our superiority, welcome to the site!

Bleeding these systems can be a real PITA. I tried many times and finally went to Napa and bought a reverse brake/clutch bleeder tool ($50 ish) You attach the tool to the bleeder on the caliper and pump fluid to the master cylinder. The air will bubble out into the reservoir. Super easy 1 person job that took me 20 minutes to do the entire truck.

My guess is you have air in the system. If it were simply a leak you would have some (very little) pressure and a mess of fluid on the ground. Once air is trapped, it just goes back and forth in the lines unless you have a pressure/vacuum/reverse bleeder. You also need to bleed the M/C first before trying to bleed the lines. Once again, an air pocket will just move around like the ol whack a mole game. It's also possible your new M/C is defective even though its new. With todays crappy made in China parts it's more common that you think.

 

[Bextreme04]

Did you bench bleed the MC before installing it?

 

[jfrancom101]

Your from Idaho. I'm from Idaho. You have an 85 k20. I have an 85 K20. You have hydroboost. I have hydroboost. Now that we have established our superiority, welcome to the site!

Bleeding these systems can be a real PITA. I tried many times and finally went to Napa and bought a reverse brake/clutch bleeder tool ($50 ish) You attach the tool to the bleeder on the caliper and pump fluid to the master cylinder. The air will bubble out into the reservoir. Super easy 1 person job that took me 20 minutes to do the entire truck.

My guess is you have air in the system. If it were simply a leak you would have some (very little) pressure and a mess of fluid on the ground. Once air is trapped, it just goes back and forth in the lines unless you have a pressure/vacuum/reverse bleeder. You also need to bleed the M/C first before trying to bleed the lines. Once again, an air pocket will just move around like the ol whack a mole game. It's also possible your new M/C is defective even though its new. With todays crappy made in China parts it's more common that you think.

Haha, yeah we got a lot in common
And that makes sense. I guess air could be anywhere in the system not just right at the bleeders. I'll probably try that out with that tool. Maybe I try bleeding normal but just run more fluid than normal and see if air bubbles eventually come out. I'll try that first and then if it doesn't work wait for my paycheck and then go buy the tool. Thanks!

 

[jfrancom101]

Did you bench bleed the MC before installing it?

Yes
I only had the plastic caps. Not the tunes and all but it seemed to work fairly well still

 

[SirRobyn0]

@jfrancom101 I'm not try to insult you with this statement, so please don't take it that way, but I have no idea what your mechanical abilities are and I seen it more than once at the shop. I guy will install in the calipers upside down, in other words the right on the left side and the left one on the right side, doing that the bleed ends up at the bottom of the caliper instead of at the top, making it impossible to get the air out. So be sure that's correct, or even a bleed tool won't help.

 

[Turbo4whl]

I have stated this before, GM front disc's, rear drums? The rear shoes must be adjusted properly or the system will not bleed without a pressure bleeder or a tool like @SquareRoot talked about. You can bleed them without the tool if you adjust the rear brakes where they should be.

You can even cheat this, by applying the parking brake, so long as the parking brake fully applies the rear shoes.

 

[jfrancom101]

@jfrancom101 I'm not try to insult you with this statement, so please don't take it that way, but I have no idea what your mechanical abilities are and I seen it more than once at the shop. I guy will install in the calipers upside down, in other words the right on the left side and the left one on the right side, doing that the bleed ends up at the bottom of the caliper instead of at the top, making it impossible to get the air out. So be sure that's correct, or even a bleed tool won't help.

Haha, no offense taken. I'm still learning the whole mechanics game but luckily I did know that. Thanks for the thought though! If you'd caught me 2 years ago that might have been an issue

 

[jfrancom101]

I have stated this before, GM front disc's, rear drums? The rear shoes must be adjusted properly or the system will not bleed without a pressure bleeder or a tool like @SquareRoot talked about. You can bleed them without the tool if you adjust the rear brakes where they should be.

You can even cheat this, by applying the parking brake, so long as the parking brake fully applies the rear shoes.

I definitely didn't know this. I didn't even touch the rear brakes thinking that the front brakes was definitely the priority. I'll try that when I bleed them today.

 

[Bextreme04]

I definitely didn't know this. I didn't even touch the rear brakes thinking that the front brakes was definitely the priority. I'll try that when I bleed them today.

When bleeding brakes you always start at the brake farthest away from the master cylinder. This will be the rear passenger side. Then do the rear drivers side, then the front passenger side, etc...

How were you trying to bleed your brakes(after changing the master cylinder especially) without bleeding the rear brakes? This alone could be why you aren't getting anywhere with bleeding your brakes.

Yes
I only had the plastic caps. Not the tunes and all but it seemed to work fairly well still

This quote confuses me. How did you bleed the MC? Most master cylinders now a days will come with the hose to bleed them, but some don't. You should have a hose coming off the lines back into the reservoir. Push in on the MC plunger to pump the fluid through until no air bubbles show up in the fluid being pumped through. If you don't do this, you will be chasing your tail forever with air stuck in the MC plunger.

 

[jfrancom101]

When bleeding brakes you always start at the brake farthest away from the master cylinder. This will be the rear passenger side. Then do the rear drivers side, then the front passenger side, etc...

How were you trying to bleed your brakes(after changing the master cylinder especially) without bleeding the rear brakes? This alone could be why you aren't getting anywhere with bleeding your brakes.


This quote confuses me. How did you bleed the MC? Most master cylinders now a days will come with the hose to bleed them, but some don't. You should have a hose coming off the lines back into the reservoir. Push in on the MC plunger to pump the fluid through until no air bubbles show up in the fluid being pumped through. If you don't do this, you will be chasing your tail forever with air stuck in the MC plunger.

Sorry for the confusion, and yeah I'll get those back brakes. It snowed like crazy today and there isn't room in my shop, so I'll have to wait for it to clear a bit to bleed the brakes but I'll do it with that sequence.

And as far as bleeding the MC. It didn't come with the hose like most of them do, it only had 2 plastic screw in caps that screw into where brake lines hook up. So I kinda used my finger and then those as a one way valve to prime it. I was finally able to town the truck home so now I have hose so I migjr want to try bleeding that as well?

 

[Bextreme04]

Sorry for the confusion, and yeah I'll get those back brakes. It snowed like crazy today and there isn't room in my shop, so I'll have to wait for it to clear a bit to bleed the brakes but I'll do it with that sequence.

And as far as bleeding the MC. It didn't come with the hose like most of them do, it only had 2 plastic screw in caps that screw into where brake lines hook up. So I kinda used my finger and then those as a one way valve to prime it. I was finally able to town the truck home so now I have hose so I migjr want to try bleeding that as well?

Yes, you will NEVER get the brakes to bleed if you didn't get all the air out of the MC. I would recommend either making your own MC bleeder or buy a kit. I have a set of old line ends I cut off a junk yard truck for doing mine.

 

[SquareRoot]

Yes, you will NEVER get the brakes to bleed if you didn't get all the air out of the MC. I would recommend either making your own MC bleeder or buy a kit. I have a set of old line ends I cut off a junk yard truck for doing mine.

Exactly! Or do what I did and picked up this handy, one man job, easy-peasy, quick, no-cussing item from Napa. You can actually bleed a dry system, (incl the MC) if you take the time. It's worth its weight in gold, especially when it came time to bleed the hydraulic clutch. You think the brakes are a PITA? lol

 

[Bextreme04]

Exactly! Or do what I did and picked this handy, one man job, easy peasy, quick, no-cussing item from Napa. You can actually bleed a dry system, (incl the MC) if you take the time. It was worth its weight in gold, especially when it came time to bleed the hydraulic clutch. You think the brakes are a PITA? lol

Its important to note if you use the handy dandy tool, you will still need to actuate the brake pedal to get the air out of the piston in the MC(not constantly, but every once in a while after you pump the fluid up to the MC you should give it a pump). I agree this thing is worth its weight in gold and well worth having in any shop.

 

[SquareRoot]

Its important to note if you use the handy dandy tool, you will still need to actuate the brake pedal to get the air out of the piston in the MC(not constantly, but every once in a while after you pump the fluid up to the MC you should give it a pump). I agree this thing is worth its weight in gold and well worth having in any shop.

True. I just used a piece of 2x4 between the brake pedal and the steering wheel to keep the MC valve open.