Okay car guys and gals, this one is for you. After a stop today I started up the bus and went to drive away only to find the brakes were locked up. Like so locked up the bus wouldn’t move. For a second I thought the tranny had gone out because we were in gear and not moving. After a few minutes they loosened up, but then they were super soft and we had almost no stopping power. They worked a bit if I pumped them a couple times.
We limped into a truck repair shop and bled the four brakes. I drove around the block, but there was no change. Super soft and needed to be pumped. We all scratched our heads, then took off the wheel that had just been worked on in Amarillo. Everything looked okay there. Then the mechanic and I rolled underneath to look at the brake booster and noticed another bleed valve on there. We bled that one and boom, the brakes felt like new again. I drove around the parking lot, all good. We left, drove through town about two miles to an RV park. I went inside to pay with the engine running, came back out and the brakes were locked up again. Shut down the engine. Waited a couple of minutes and they loosened up, but were soft and hardly working again. Limped into our spot and called it a night. Long day.
My only thought is that since I drove around the parking lot between bleeding the four brakes and the time we bled the booster that maybe there is still air in the system. I need to bleed the whole thing at one shot. Of course it could be something more, master cylinder, or booster? I don’t know. Any ideas?
42 Comments on “Brakes”
check vacuum hoses to the booster
I like that matching wrecker.
If it was a master cylinder brake pedal would be dropping when pressing on it.
I would bleed it again starting with the nearest brake cylinder.
http://my.cardone.com/techdocs/PT%2051-0001.pdf
hydrovacs have filters….let me look into this…:)
http://dave78chieftain.zxq.net/HydroVac_brakes.html
https://www.youtube.com/watch?v=keChCevuKk8
If raw gas or gas fumes reach the rubber diaphragm, it will deteriorate quickly. On cab-over installations with no fire wall, it may be necessary to make a bracket on top of the air filter, so that the vacuum line can go up for eight or 10 inches, and then down to the power unit.
The vacuum line from the manifold to the unit should first go to a check valve that has been checked to be sure it’s working properly. The vacuum line should then be routed directly the booster.
Pat,
The master cylinder is not releasing pressure, most likely not enough free play between the pedal and the master cylinder. (When that happens the compensating port is not open to allow pressure to release)
check simple solutions a plugged air filter/breather ?
Perhaps during the carb re-install….a vacuum line/hose was swopped….to the wrong place ?
Years ago, we had a similar issue with our 75 Ford – the brake lines had somehow, with movement, got “pinched” with a specific movement of the vehicle. They “failed” going down a long hill outside Kelowna, BC. Our Airstream trailer brakes slowed our descent.
We had extensive brake work done before our trip in Vancouver, BC. When we landed in Kelowna, the mechanic there said the failure was due to a brake line not being secured where it was supposed to be. This mechanic traced the line to find the problem.
Hope you figure it out.
Brake Master Cylinder
A visual inspection of the entire hydraulic system should be performed to rule out any external leaks. Look at all the connections including the calipers and wheel cylinders. If the vehicle has no external leaks, suspect and inspect the master cylinder.
Brake Master Replenishing Port
The master cylinder consists of two pistons that work in tandem to apply pressure to the braking system. These pistons use cup seals to seal fluid in the pressure chamber. As the seal moves past the replenishing or compensating port it begins to create pressure in the pressure chamber, through the lines, and onto the brake units. A return spring forces the piston back to its resting position. This action allows the brake fluid to return from the brake lines and piston chamber into the reservoir. If the brake pedal is over adjusted or there is debris blocking the replenishing port the fluid will not be able to return to the reservoir. This can result in residual pressure in the braking system. This residual pressure will cause brake drag and lock up. Loosening the flare nut at the master cylinder port will often relieve this excess pressure, but will not fix the underlying cause.
If the fluid bypasses the cupped seal on either the primary or secondary piston, the pedal will drift to the floor and you will have to continually pump the pedal to maintain pressure. This is a common problem with master cylinders on all types of vehicles. These seals can wear in time or become contaminated. The primary piston cup seals are located on the pedal side of the master cylinder and the secondary the front of the vehicle. An O-ring is located at the rear of the primary piston to prevent fluid from leaking past and into the brake booster.
You have gotten great advice. I could suspect a Hydrovac booster… or any vacuum brake booster even if your RV pre-dated Bendix’s Hydrovacs. The master cylinder piston actuator rod length adjustment is important, but probably doesnt cause problems once the brakes have been working well – unless someone adjusted the piston actuator rod, or maybe something around the brake pedal linkage to the master cylinder is interfering with the brake pedal begin fully “returned”; and the compensating ports therefore not being opened. On my dodge based 1972 motorhome there is a nice strong spring that pulls the brake pedal to its ‘returned’ position. That spring finally broke one day, due to rust/wear on one of its ends. I used a bungee cord to pull the pedal all the way -up-, until I found a hardware store that had a good set of long strong springs.
Another cause of brake annoyance: Brake rubber hoses can deteriorate from the inside after enough years, and not allow free flow of the fluid; Even if its not the cause of your present brake problem, I would recommend replacing the brake hoses to the front wheels and to the rear axle, unless you know they have been replaced in the last ten years. I just replaced a set of front brake hoses on my 27 year old Chevy van. With the old hoses I took off the vehicle, I wasnt able to blow through them.
There are companies that rebuild those Hydrovac boosters, but I dont know the names anymore.
And the business of the brakes freeing up after shutting down the engine is another known symptom with brake boosters. The vacuum in the intake manifold drops to zero when the engine stops. The one-way valve in the vacuum line from the engine to the booster should keep a vacuum in the brake booster for a while, BUT with a sticking one way valve, a leaking vacuum line, a brake booster diaphragm that is torn, or a bad booster operating valve; can let the vacuum bleed down right away – and let the brakes free up.
P.S. You can have age cracks in the fuel hoses coming forward from the fuel tank or tanks – that wont leak fuel out, but will let air get sucked into the lines with the engine running. After I had installed a fuel pressure gauge, I found I had to replace those fuel hoses, to keep up good fuel pressure; along with replacing fuel filters as they got silted up.
I get the feeling you enjoy this sort of challenge, don’t you? Good luck and hope you’re back on the road soon!
Did you remember to release the NAPA break light switch? JK
So basically what I’ve determined from the advice I’ve gotten here and on Facebook is that it could be the brake lines, the brake booster, or the master cylinder. Ummm, that’s the entire brake system you guys. Thanks a lot. 🙂
You are welcome 🙂 Assuming all the lines, vacuum & pressure, are OK, one new master cylinder & one new vacuum booster unit coming up.
Paul T
Yep.
The only reason I’m not suspecting the booster is that the previous owner just had it replaced a month before we bought it. I’ve got the receipt. Not that it couldn’t be faulty, but…
Did they buy it from NAPA?
Yep two problems that seem to be at odds with each other. Dragging or locked brakes usually means fluid not returning to the master..block return port if all four wheels , collapsed/pinched line if one.
But soft brakes that fix with pumping usually air in the lines or bad seals in the master.
Guys above seem to have more detailed info than I.
I’ll throw something out that is a ridiculous long shot … still brake line related though. Is there some place where the brake lines can get heated? Maybe some exhaust hitting it or laying too close to something that gets hot? The lines and fluid could be getting way too hot and, when you stop for a bit they overheat and pressure up – thus locking up after a stop. Bleeding releases the pressure and all seems well until the lines heat back up again. If you have age cracks like Dave Denison describes, air and maybe a little water can get in there making this problem worse.
Be aware, I have no idea what I’m talking about. 😉
Vapor.
start with the easy least expensive spot and work your way through it
good luck
stan
I read through some of your earlier posts and believe Larry at 10:32 has this one pegged! A leaking line or bad wheel cylinder will show oil somewhere to give you a clue. A bad brake booster of any kind (doesn’t matter if it is hydraulic, vacuum, electric, or air assist) will just make the braking harder to do………not impossible! Boosters run independent of the master cylinder. That only leaves the master cylinder! When the piston seals go bad fluid blows past them making very strange things happen! Every time the oil happens to blow past the seals the entire system is going to do one of two things. (A) oil blows past the pistons, pressurizing the wrong side of the piston and locking the brakes down or (B) oil blows past the pistons and the system looses pressure and the pedal goes to the floor and has to be pumped up again……..my two cents!
By the way, the brake light switch is not working because of char on the switch contacts. Those switches are not designed to carry any kind of unregulated amperage load.?.?.?. The switch is made to operate a small control relay that will handle a higher amperage! When you have problems with old lights shorting or blowing out or even adding extra lights to an already overloaded switch it chars the contact closure and when you move the terminals around it will act like it is working properly! Your brake light issue is a 10 dollar fix!
But so far, everything mentioned I would consider as normal problems and maintenance for any truck as old as your RV. So far everything mentioned has happened either to my vintage Winnebago or to RV brake systems on friends vintage RV’s & trucks.
Problems with the rear brakes happen also but are more work to get to. You might want to try pointing your infrared temp sensor at both rear brake drums and both front brake drums the next time you brake to a stop in a good location. They should basically all be about the same temperature. After a long drive, all six tires should show br at equal temperatures too; the ones on the sunny side being a little warmer though.
Im still waiting to hear if/when the bumfuzzle bus has front wheel shimmy, or drifty steering response…. Those can seem mysterious but are all fixable too.
I wonder if your parking brake is incorporated into the rear axle brakes, or is it the little drum mounted on the rear end of the 727 transmission. Getting those to work right is a discipline in itself.
Incidentally – there will be light at the other end of the tunnel. All of these mechanical problems are worth fixing.
hmmm… Hulk action figure jerry-rigged brake light switch, there’s the problem. 😉
I had an old Fiat were happened the same.
In my case, the rotor was too thin (or the pliers too open due to wear or both) that the pads would move and be constantly touching the rotors with a certain pressure (you don’t fell it when driving), leading the entire system to heat up to the point of boiling the brake fluid causing the “air” thing …
Ricardo
Pat:
If it were me, I would focus on the Master Cylinder.
firstly, you should not have been able to previously take that pressure switch off with out fluid leaking out, and having to bleed the system.
In a larger system like you have you may also have to bleed it more than once, if you are doing it your self these are also very helpful at a reasonable price, http://www.speedbleeder.com. they allow you to bleed it on your own very easily.
I have had boosters go, and have never had this issue.
In my minds eye, you recently bled the system and replaced parts. This could have made the master cylinder work at a slightly different position with-in the cylinder, which may have been worn a touch allowing for blow by in the cylinder creating back pressure.
After letting it sit for a bit the pressure releases a bit and you can continue on.
Keep us up to date with your progress
Isn’t your brake light switch in the master cylinder?
If so, you now have two failure symptoms that relate to the MC.
If you take a look at the carb photo in the cadillac ranch folder you can see
the big hose has no clamp, none of them do, this is where your leak is that is
allowing air in your breaking lines.
i’d replace brake master cylinder and all the hoses. the hoses might be swollen.
All of the suggestions here(from what I’ve skimmed) are not diagnosing the problem. The good thing here is that the brakes are remaining “on”, to allow a diagnosis(not a part swap diagnosis). The first thing here is that in order to hold a vehicle of that weight, the hydraulic system has to be able to build a lot of pressure….so forget leaks or air in the system! Focus on what could hold the brakes on. The master cylinder is not being disengaged for some reason, be it vacuum or hydraulic assist failure. Easiest to try first is while the RV is being held, disconnect the booster vacuum line after the check valve and see if the brakes release. I guess you have a restriction in that vacuum or hydraulic system, not allowing for the bleed down. Just my 2cents. Good luck
or parking brake….
Pat do you carry a fire extinguisher or two?
Aaaahhh! The joys of the classics…. I hope you figure it out painlessly(at least no blood).
Just think back to all of those wonderful hours(days) on the busses of Mexico…My attempt at pain relief:-)
Nice color matched wrecker. Enjoy the Ride!
Wow I’ve never seen such all over the board advice. The key to your problem is locking brakes just sitting. Now when its freed up and you go back into rig, can you roll the rig without touching the brakes or on its own without touching anything it locks up?
*If locks up on its own sounds like heat is getting to the brake line
*If locks up when depressing the brake to place in gear sounds like master cyl.
Like in a boat all your pumps (bilge, sea water) all need to be in perfect order, so does the complete hydraulic braking system, master/wheel cyl/brake fluid/marginal brake lines. Brake systems don’t age well, especially going from no or limited use to everyday use.
I know this is not what you want to hear, but given the size of the rig you really need a shop to go in and replace the remaining 3 wheel cycl (you changed one already), master cyl if original and you stated booster was replace so leave that, good fluid flush, complete bleed and now at least you have a complete system is a state of good repair.
I’ve owned plenty of old cars and boats, both with nickle and dime you to death unless to really go in and completely go through system by system.
Does anyone know if this is a dual reservoir master cylinder?
If it’s not then that would be something I would upgrade as well. I did this on my old 46 that was only single.