The Stovebolt Site Forums The Shop Area Engine Shop What causes brake fade?

When I start out, I can stop on a dime and leave nine cents change. When I get in stop and go traffic the brakes get softer and softer. After a little while I can't lock them up if I had to. I've got drums all around, new pads on front, dual master cylinder with 7/8" bore, 7" or 8" booster. Help!

beltfed,

It sounds scary. Does the pedal stay firm but the rig doesn't slow down, or does the pedal go spongy and 'no-slo'! I think if you have a good, firm hard pedal but 'no-slo' it's a lining issue. About the only enemy of a modern lining is heat. If the brakes were adjusted too tight and were creating heat as you drove, they soon would overheat in traffic and your stopping ability would go down the drain.

I think this could happen without the usual smell associated with cooking brakes. You mention new linings on the front...how about the rear! If only half the system is trying to do the stopping, it will fail pronto. I don't see what kind of rig we are talking about in your profile...probably a 1/2 pickup. If you have a neat IR temp gauge, shoot all the wheels next time it happens...you might find out something interesting.

Stuart

The pedal will go to the floor it I stay in heavy traffic. It get somewhat spongy but mostly as I stop more and more it get closer to the floor each time. I notice this is happening and start leaving lots of following distance.

A leaking booster will do that to ya also.

beltfed,

I wouldn't call that brake fade. I think that is a mechanical problem involving the master cylinder. It sounds like repeated applications in a short time period aren't letting something recover. It doesn't sound like the system is leaking externally and if you have firm, high pedal when you first start out, there isn't air in the system.

It almost sounds like the master cylinder is aerating the brake fluid somehow...it is generating it own 'spongy syndrome'. Does this sound like it is possible.

Stuart

Sounds like your brake fluid is boiling at the drums. May want to upgrade to a higher DOT (3 maybe) rating and a good name brand. Also, you could wrap some large screen door springs around each drum for better heat dissappation. Every little bit helps,
Scott

If the pedal is going to the floor as you sit in traffic,the fluid is slipping past the "cup' in the master cyl.
In my past experiences with "fading" stopping power,it has been because of drums that were turned past their safe allowances. It causes them to heat up waaay faster than normal.
You definitely have brake problems that should be looked into. Especially if you have your chilluns in the truck with you.
Marty

If you look at the DOT 3 brake fluids, you will see the boiling temps are all over 500&deg...I can't believe an old bolt cruising in traffic could get there..not without smoke anyway.

Stuart

See if I can ansawer the above as I need to get my arms around this. Fluid levels in both reservoir are holding and not creeping down, already have springs around front drums and rears are late modle chevys with fins, already use DOT 3 fluid, nerver turn the drums so don't know if they are out of specs, vacuum to booster is about 15 to 18 and hose looks good. What wrong with these darn brakes?

I have seen a MC adjusting improperly so that the brake pressure can't fall after release of the pedal. The result was dragging brakes which fade fairly quickly.

Hmmm,,Thru all of these posts,we still dont know what year your truck is, and the brake mods. that have been made to it. The answer could be right there................
Marty

When was the last time you changed that brake fluid? Was it from a new can? It absorbs water from the air and that lowers the boiling point very quickly.
Also, if you have new shoes they probably aren't bedded to the drum properly and aren't making enough contact area to stop effectively.

Also check your master cylinder pushrod adjustment to make sure they release properly.

I've never concealed the fact that this one is a 1939 Packard model 110 coupe series 1288. Now, how an upgraded like brake system for a stovebolt truck is different than a Packards 4 drum, vacuum booster with dual master cylinder is a msytery to me. I seldom mention that it's a Packard as someone might get bent out of shape that it's not used to haul things other than people.
Truckernix, how do you check the MC to check for pressure release. I adjust for about 1" to 1-1/2" free pedal at top of stroke, adjust pads for very slight "scraping" as I hand turn the wheels. I have the CAR up on stands. I will start it, apply brakes, release them and check for binding by hand turning the wheels. Will that do it? Thanks.

Here's my $.02. Go out & make it do what you are describing, jack it up immediately & see if any wheels are dragging-it sounds to me they may be dragging(poss. only after repeated stops). If any wheels are dragging, open the bleeder on that wheel & see if the dragging goes away, if so-problem is in that wheel, if not-master is keeping some pressure on or a bad brake hose(or a clog in a line) elsewhere is keeping pressure on. Crack the brake line loose, line @ master loose, etc.,,just go backward thru the system until the wheel turns freely. Linings get hot, then don't wanna stop. Also poor quality linings could be to blame, OE asbestos are hard to find these days. Some cheaper replacements jsut won't stop. Or poor contact, some shoes are arc-ed for turned drums, look & see how the shoes are wearing, should be even contact. Shoe return springs good?

Correct me if I'm wrong Fred, but I think what you are referring to is the pushrod stop on the AD Master Cylinder. It is possible to put the stop in backwards and this blocks off the compensating port which relieves the pressure when you release the peddle by holding the pushrod and in turn the piston further back in the bore.
Beltfed, what your describing doesn’t sound like “brake fade”. Brake fade is typically caused by heat and I don’t think you are doing the kind of driving that is going to generate that kind of heat. Everybody’s got their opinion and of course, I have mine. I think you simply have a bunch of air in the line and no matter how hard you pump things up all your doing is standing on a column of air. I would completely flush the lines with fresh fluid and pay extra attention to the bleeding process. Even if you have already bleed the brakes previously, you may have a small leak that has sucked air back into the system.
I have no idea of what the physical layout of your MC is on a ‘39 bastich but its all ready been suggested here several times, that it could also be fluid leaking past the cup in the MC, that would be my second guess.
Denny Graham
Sandwich, IL

"Fade" is caused by overheating, period. The symptoms are a very firm pedal, and no stopping effect. If the shoes are putting heat into the drums faster than the airflow around them can dissipate it, there is plenty of pedal, and no stop. That's usually caused by heavy loads, and/or long downgrades at too high a speed.

If the pedal is getting lower as you drive, there's either air in the system you haven't bled out, or air is leaking past the M/C piston as the brakes are used repeatedly. I'd suggest rebuilding or replacing the master cylinder, and a full flush and bleed to replace any moisture-contaminated brake fluid. You can go as high up the chain as DOT 4 fluid, but don't use DOT 5 unless you're willing to rebuild the whole system, wheel cylinders and all. DOT 5 is silicone-based, and isn't compatible with lower grades of fluid.
Jerry

I think we need a better description of the problem.

Is it like HRL described above, hard pedal, not stop?

Or, soft spongy pedal?

Or pedal goes to floor?

OR??????

Hotrod has it right. Anyone who drove pre-disc brake work trucks knows you could stand up on top of the pedal, push the back outta the seat, whatever, & that sucker would stay right up on top & do Nothing! Very scary. I wouldn't even worry what the fluid type or age is. You've got a pressure problem if the pedal goes down. Get another M/C & be sure the system is bled. Be sure a flex hose isn't de-laminating & 'ballooning'. A dual M/C is a bit harder to bleed sometimes. Doug

Denny,
my personal experience with the problem was with a 75 camaro that I had from new. I had the MC/booster replaced at a dealership and the brakes overheated on the way home, about 1 mile! When I got out, I could smell the glue. I took it back and they adjusted the linkage for the MC.

If the car has a MC for a disc/drum setup, there may be no residual valve for the front system. That may take a separate valve. You are sort of in no man's land with a partially updated brake system.

You say you have a power brake unit. What kind of manifold vacuum does your engine have? Could it be that the vacuum is marginal and the booster/reservoir bleeds down in stop and go traffic? I've never heard of that causing a 'spongy' feeling, but . . .