ok its happened to me before but today it was extra scary. ill have my truck opened up on the free way and go to slow down and find that my pedal goes to the floor and the brakes barely drag.... very fun at off ramps. they come back up after you pump them a couple times... this doesnt happen around town it only happens on the freeway. anybody know whats up? ive already replaced the booster and the master cylinder and ive blead the system numerous times.....
Put the proper residual valve(s) in your brake system. They keep the proper 'set' in your brakes so the pedal will be right when you hit it after a run down the freeway. Either that or you have a bad master cylinder...but I suspect a lack of residual valving.
are those the same as proportioning valve? cause its got the one out of the car that i got the rest of the system out of... it wouldnt suprise me if it was the master cylinder it would be the 4th bad rebuilt ive gotten from kraigen
I always buy the rebuild kits and do it myself......that way I know either 1. it's done right because I did it myself OR 2. I screwed the pooch and it's my fault I rearended that lil ol lady in the slow lane.... xxx Brandy
The residual pressure valve keeps a small (residual) amount of brake fluid pressure in your brake lines, instead of letting all of your fluid (and pressure) drain back to your M/C. This will give you a firmer pedal and faster braking action. Check with Godwin or some other brake component company. Unless, of course, you enjoy making this face !
When you bleed your brakes, are you getting more air out of the fronts than the rears? Do you have disc brakes on the front? A short run down of your system and vehicle may shed some extra light on the problem.
One thing you might not have thought about... Maybe your brake fluid is overheating? Is there a source of heat too close to your lines or mc? r
Think that would be more of a problem in town than it would be on the freeway. Sounds to me like a bum master. The pressure valve is an interesting thought as well. Is your fluid staying full? When you bleed your brakes how many times do you cycle it? Maybe try using a vacuum bleeder.
if it was overheating wouldnt he get MORE drag ??? as its expanding? I know sometimes when the van gets real really hot on a hot day....mine heats up jsut enough to keep enough pressure on the brake light switch.......
This is the theory I'm working with, My thoughts are old fluid and some freeway heat causing the lines to steam up and create air pockets. I guess I would like to know if this is a recent problem on an otherwise reliable system or something that was created since the last time something was changed.
its a whole braking system out of a 70's ranchero disc front drum back its in my 55 f100. its not a recent problem either its happened a couple times but its only happened on the freeway and i dont take the free way much so i kinda forget about it because it stops fine in city driving. it might be the heat thing because now that i think of it my muffler is kinda close to my rear brake lines. i looked into residual valves and found wilwood ones in jegs for 16.99 each i guess theyre worth a try. it says to mount it bellow the horrizontal plane of the caliper.... im not sure what that means. im also going to get a new proportioning valve and check my callipers and wheel cylinders for leaks... i hope i get this fixed going 90 and realizing you dont have brakes is no fun....
Where's your sense of adventure? I drove all last season without residual pressure valves....scary. Added them this year and the 'ol bucket stops on a dime!
i kinda like to be able to stop.... i lost my sence of adventure trying to figure out how i was going to make the turn on the freeway exit going 80
If the Master cyl is under the floor, or below the level of the calipers/wheel cyls, then residual valves are a good idea. They work like the others have said. If the master is on the firewall, above the calipers/whl cyls, then you really don't need them and have a different problem.
I run into this with my fairlane sometimes if I have been on the interstate for a couple hundred miles non stop. my system is all factory spec but the master (mustang dual circuit) with 4 wheel manual drums. I try to pump the brakes every 45 minutes or so to keep it up at a nice level. everything is fine in the system but the master is about 3 years old I think and an advance reman part....everything else is the same as when I pulled the car from teh field aside from a wheel cylinder in the rear, rear flex line (changed to braided) and a couple hard lines.
The residual pressure needed to maintain pedal should be automatic when the master is above the calipers/whlcyls. I read of this years ago about brake fluids. You aren't using silicone fluid are you? The boiling point of silicone fluid changes significantly with altitude and atmospheric changes. I see you're in so cal. Pretty far above sea level? That being said lemme say I'm going on memory here about the pressure deal and silicones. A pretty old memory. Beyond all that, look closely toward the heat thing. Our beloved rods get plenty hot when driven at high rates of speed and that of course means all of it gets hot especially the exhaust. "it might be the heat thing because now that i think of it my muffler is kinda close to my rear brake lines." I've had mufflers glowing red on freeways. I obviously don't know your whole get-up on the truck, but if at all other times it's OK, I'd look at the heat. I hope this helps and if ya get 'er figgered out let us know.
yeah im thinking more along the lines of the heat thing now.... a big block probably gets pretty hot after about 5 minutes and just under WOT... and now that i think of it more my exhaust turn down points at my rear axle where my brake line is.... could that be the cause of it?
Good topic. Mine did the same thing on the way home from Paso. Dual master with manual drums all around. Running residual valve per Master Brakes info. When I left in Paso brakes were fine cruising back and forth in town. Went from Paso to Santa Barbara non stop. Had to pump like 5 times but they still felt mushy. I just figured it was the heat also. Maybe not?
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