Soft brake pedal when not running?

I looked at some trucks in Frankfort today and on a couple of them the brake pedal would easily push all the way to the floor with little / no resistance. Others felt firm.

The ones that had no resistance, the trucks would not start (dead batteries) so I could not give it a chance to build up air pressure and see if that made a difference.

If you were planning to drive a Deuce off the lot, would this be a deal-breaker?

with or without air, the pedal should be firm!!!!

Going to the floor means either air in the lines, bad MC or maybe wheel cylinder or a leak somewhere, possibly even brake shoes in bad need of adjustment. Even a combination of all of the others listed. Did the pedal "pump" up and get firm, or was it mushy every pump?

I wouldn't call it a deal breaker, but you would be better to have a tow truck to get you to a parking lot where you can work on the brakes to try to fix the problem. Then as a back-up plan have a hauler at the ready in case you can't fix the problem with limited tools/time.

I can't stress this enough....DO NOT DRIVE IT ON PUBLIC ROADWAYS UNLESS YOU CAN GET THE BRAKES WORKING PROPERLY

Without brakes, you are just a big accident waiting to happen! Better to have it hauled and avoid the courtroom drama.

I have a little battery powered inflator which weighs about 5 pounds. I used it to pressurize the master cylinder to about 10 PSI (regulator from Home Depot, 1/8 and 1/4 NPT fittings, 1/4" poly tube) which forced brake fluid into the wheel cylinders, making the brakes effective.

Why does this happen? Some trucks have check valve breather caps, these let air out but not it. With changing weather or brake fluid seepage, negative pressure can build in the master cylinder reservoir. This causes the piston boots to slightly retract in some of the cylinders. If this loss of displacement is greater than the master cylinder's delivery volume (when brake pedal is depressed), you will get a soft pedal. Also, vacuum bleeding can do this, TAKE NOTE!!!

Anyhow, after pressurizing the master cylinder to about 10 PSI, all the wheel cylinder pistons were pushed out to their stops and brakes were restored!

FYI, the fitting on the MC cover is 1/8 NPT.

Something else to consider is that the hydraulic part of the system is in great shape. (I agree with the above threads that you should check every part real good before moving the truck.) Yet, you get no pedal because the brake shoes are worn and have never been adjusted. If all 6 wheel cylinders have to move 3/4" total before the shoes touch the drums, your little 2" stroke master just doesn't have enough stroke to make it work. Unless, the air pack is full of air and helping out instead of being pushed backward itself when the air is gone taking up even more fluid.

Read about major and minor shoe adjustments, do them, check all the hydraulic parts, fix anything you find wrong and you should have brakes that will let the rear lock up with just a toe twitch. If the truck is empty that is.

Bleed brakes. 5 dot fluid. extra person to pump peddle, funnel to add fluid.

Keith_J said:

I have a little battery powered inflator which weighs about 5 pounds. I used it to pressurize the master cylinder to about 10 PSI (regulator from Home Depot, 1/8 and 1/4 NPT fittings, 1/4" poly tube) which forced brake fluid into the wheel cylinders, making the brakes effective.

Why does this happen? Some trucks have check valve breather caps, these let air out but not it. With changing weather or brake fluid seepage, negative pressure can build in the master cylinder reservoir. This causes the piston boots to slightly retract in some of the cylinders. If this loss of displacement is greater than the master cylinder's delivery volume (when brake pedal is depressed), you will get a soft pedal. Also, vacuum bleeding can do this, TAKE NOTE!!!

Anyhow, after pressurizing the master cylinder to about 10 PSI, all the wheel cylinder pistons were pushed out to their stops and brakes were restored!

FYI, the fitting on the MC cover is 1/8 NPT.

Click to expand...

Thanks, I'll prepare for this possibility. First I'll show my ignorance: what's a regulator?

I have a little inflator that plugs into the cigarette lighter. I could either park close enough to use that, or find a way to wire it to 1 of the 2 batteries I'll be bringing along...

But my inflator has no way to connect a different nozzle. Maybe it won't work for this.

JCKnife said:

Thanks, I'll prepare for this possibility. First I'll show my ignorance: what's a regulator?

I have a little inflator that plugs into the cigarette lighter. I could either park close enough to use that, or find a way to wire it to 1 of the 2 batteries I'll be bringing along...

But my inflator has no way to connect a different nozzle. Maybe it won't work for this.

Click to expand...

The regulator is a device that allows only a certain amount of air pressure to build up in any down stream components that you have hooked into it....in other words...if you have a compressor full to lets say 100 PSI, install a regulator in the output hose and set it for 50 PSI, and all of your air tools and equipment hooked up after the regulator will only see 50 PSI.

However, as nice as a set-up like this is, I still prefer the old fashioned garden sprayer pressure bleeder. Simply because it has a built in reservoir (the sprayer tank), so you don't need to unhook everything after every wheel to be certain the master cylinder is full.....it refills it as you bleed the brakes.

Read the entire thread here...M35 Brakes. Warning signs ignored could have been serious.

My post about an easy to build bleeder here....Bleeder

Speddmon said:

The regulator is a device that allows only a certain amount of air pressure to build up in any down stream components that you have hooked into it....in other words...if you have a compressor full to lets say 100 PSI, install a regulator in the output hose and set it for 50 PSI, and all of your air tools and equipment hooked up after the regulator will only see 50 PSI.

However, as nice as a set-up like this is, I still prefer the old fashioned garden sprayer pressure bleeder. Simply because it has a built in reservoir (the sprayer tank), so you don't need to unhook everything after every wheel to be certain the master cylinder is full.....it refills it as you bleed the brakes.

Read the entire thread here...M35 Brakes. Warning signs ignored could have been serious.

My post about an easy to build bleeder here....Bleeder

Click to expand...

I do have a backpack sprayer. I'll read up later--thanks!

Speddmon wrote:

I can't stress this enough....DO NOT DRIVE IT ON PUBLIC ROADWAYS UNLESS YOU CAN GET THE BRAKES WORKING PROPERLY

Click to expand...

Thank you for saying this Speddmon

I cringe when I occasionally read about people blocking off blown wheel cylinders, taking other risky shortcuts or even joking about how green always has the right of way and/or compact cars are speed bumps.

I personally feel that there are people lurking out there that would like to see our rights to purchase MVs from GL stopped. IMHO our best defense is for us to preach and practice safety on SS and on the road.

End of sermon.

Oh heck no, I wouldn't dream of it. Close as I live to Frankfort, I could have it towed for a song.

And I'm not even a very good singer.

I have a question about DOT5, brakes and oil: With DOT3, if you get even the tiniest bit of oil in the brake system, the rubber parts will swell up to gargantuan dimension, and the brakes are toast. But is that a function of the rubber parts, or the DOT3 fluid?

Things like garden sprayers use a squirt of common motor oil to lube their pumps, and vasoline to lube the O-rings in their wands and valves.

What happens to the rubber parts, with DOT5, if you get some motor oil in the mix?

-Chuck

DOT3 is an oil...period DOT5 is silicone.

That being said, certain types of rubber do not react well in the presence or oils and do swell like you mentioned. Any braking system that was designed to work with DOT3 will work with DOT5, but you cannot mix the two together or they will form a gunky type of sludge and will render your brakes inoperative.

The drawback to DOT3 is the fact that it is an oil and as such can "draw" water from the air...that's why you always keep it in a sealed container. Silicone brake fluid resists this action much better, and has better heat characteristics and other properties making it more desirable for high performance braking applications.

As for the garden sprayer components you mentioned, if you are building a pressure bleeder you are not going to use the wand, so you don't need to worry about those o-rings. The pump, probably isn't enough oil in there to worry about mixing with anything, and the DOT5 should not have any affects on the rubber components at all since it's not an oil.

Speddmon said:

DOT3 is an oil...period DOT5 is silicone.

Click to expand...

DOT3 is NOT an oil. It is a mix of poly glycol ether, and diethylene glycol... one a glycerin based ether, and the other a glycerin based alcohol. It has more in common with antifreeze than any petroleum oil.

DOT5 is mostly poly dimethyl polysiloxane. I am guessing it is like kerosene with some of the carbons replaced with silicons.

Does anyone have any experience with DOT5 brake fluid systems and petroleum oil?

-Chuck

Hmm, no longer my problem--someone bid that truck up higher than I would go on it.

Thanks, though!