mustrmrk

I've got a little RV built on a 1987 Toyota one-ton cab & chassis with a 22RE and an auto trans. I have a brake issue I'm hoping I can prevail upon some of you smart people to try and help me out with. I found a posting on Pirate 4x4 with the same symptoms, but no follow up posts to help me along. Here's the post: https://www.pirate4x4.com/threads/ha.../#post-7524534

When I press the pedal slowly, the brakes work great - good power assist, easy stopping. When I press it quickly the pedal is rock hard and I have to just about stand on it to come slowly to a stop - no apparent power assist.

Below is a list of everything I've done so far and I was wondering if any of you have suggestions as to what to try next. I'm fresh out of ideas.

The first thing I did was pull the master cylinder and and booster and found the booster was full of brake fluid, so I replaced both. The booster is a Cardone 53-2776 dual diaphragm rebuild, and the MC is a $50 Dorman M39996. I also replaced the 3/8 fuel line that was being used as vacuum hose between the manifold, check valve, and the booster with 11/32 Gates vacuum hose. All this helped, but I still have the slow press, great power assist, fast press no power assist problem.

Yesterday I double-checked the free play on the pedal linkage and the adjustment between the booster actuator pin and master cylinder and it all seems fine.

I don't know enough about how a dual diaphragm booster operates to know if it's even possible for a booster to go from great power assist to no power assist depending on how fast the pedal is pressed, or if it has to be something else in the system.

I sold this RV (with good brakes) to a young couple a few years ago and recently bought it back. Just before I bought it back, the guy repacked the rear bearings, installed new seals, replaced the rear brake shoes and had the drums turned. He's an aircraft mechanic with the Marines, so I assume he knows how to turn a wrench. I have not pulled the axles to check his work, but the emergency brake works great and when I do the slow pedal thing the rear brakes work fine.

Someone else suggested replacing the LSPV. With this in mind, I did a temporary LSPV delete taking the LSPV completely out of the system, but still have a hard pedal on a fast press.

Just for yucks, I drained the old booster and swapped it back in. Exactly the same symptoms, just doesn't work as well as the rebuilt one, so I put the rebuilt one back on.

Unfortunately, when I pump, then hold the pedal down on the old booster and start the engine, the pedal drops some, as expected. When I do the same with the rebuilt booster, the pedal doesn't drop. So now I'm back to square one. Could I have two bad boosters? Why would the old one test good, the new one test bad, yet both show exactly the same hard pedal / soft pedal symptom? Or is the new one testing bad only a symptom of some other problem? And the one that tests bad works better than the one that tests good on a slow press. Crikies.

I put a vacuum gauge on the manifold before the check valve - it pulls 17 inHg at cold idle, then 18 after I rev it once. So I move it between the booster and the check valve. The vacuum again sits at 17 inHg at cold idle on startup. I rev the engine once briefly and it comes up to 20 inHg and sits there. FWIW, an engine compression on all 4 cyls ranges from 162 - 165 psi, well within factory spec. I shut it down, wait an hour, pull the vacuum hose off the booster, and I hear the air sucking in. I believe I have enough vacuum.

So I drive a bit, make sure to let up on the throttle a few times to build up a good vacuum, gauge still says 20 inHg. A slow press gives good assist and good brakes, and the vacuum drops to 17 inHg while I'm holding the pedal. I get it back up to 20 inHg and do a fast press, get no assist, lousy brakes, and the vacuum goes to 18 inHg. I have no idea what this means. I pull the vacuum line and the engine dies immediately. I even removed the check valve from the system and tried again - same issue with the pedal.

Aargh!

I haven't seen any kinks in the hard lines, all 4 wheel cylinders bleed fine, the pedal is high and firm with the engine off, as I would expect with a well bled system, and the flexible lines all look good.

What else could cause a hard pedal on a fast press, but allow the brakes to work well on a slow press? Flexible brake lines breaking down internally? Improperly installed rear brakes? Improperly bled MC? Maybe the new POS Dorman MC is bad? I'm fresh out of ideas.

Any constructive suggestions appreciated - thanks.

swampedout

youre not losing any fluid?
Is the brake pedal coming back up when you deprrss it?

mustrmrk

Not losing any brake fluid, and, when i depress it the brake pedal comes back up. If I'm driving and I press it fast, the brakes suck, then I let up and press slow and they work great. Makes no sense, right?

swampedout

Well yes and no.

Fast braking demands more power/force so it might expose a problem that a more gradual brake wont pick up. Whatever leak or vacuum issue is fine until pushed to a certain point.

What other brake worl have you done? Is the master cylinder good? Usually mushy brakes are in the lines, calipers and drums. A hard pedal does point to the booster and vacuum. So be very methodical with how you troubleshoot. Dont assume you got a good booster from the parts store.

wyoming9

iTrader: (1)

I have found the quality of Rebuilt parts is hit or miss .

Then boosters tend to work or not at all giving the hard pedal at all times.

The LSPV should not effect pedal pressure going hard or soft just the amount of fluid to the rear brakes

I also need to wonder just because some one turns wrenches on air craft does not mean they are good with rear brakes.

First thing I would wonder are the rear brakes binding or hanging up on what is a panic stop or hard application.

Might also be a Caliper is not working like it should and binds on a hard application.

Both being hard to trouble shoot unless you live and breathe these brakes .

Does this have drum brakes up front or was that just a slip that all 4 wheel cylinders the bleeder valves worked.

Just a thought best of Luck

mustrmrk

Thanks for your replies, swamped out and wyoming9. I appreciate your help.

I spent yesterday getting the LSPV back in line and I guess I'll be pulling the axles and checking the rear brakes sometime later this week.

​​​​​​I'll check back in if I get it figured out.

mustrmrk

Oh yeah, forgot to say that it's drum rear and disk front. Good catch wyoming9.

wyoming9

iTrader: (1)

With the miles on these Trucks and living where I do in the great liquid Brine belt I never had a LSPV that worked so first rear brake job off it came..

A few got manual valves a few nothing I never could tell much difference.

In my youth we never had brake valves you just drove suitable for conditions .

Rear brakes check to see if the backing plate might be flexing any thing to cause things to bind.

Another thought though you were up close and personal when you changed the booster the brake pedal bracket and linkage might that be binding on a hard application

Best of Luck I will be waiting for what you find.

mustrmrk

I'm with you there. I'm getting too old to be crawling around under this thing...

I'll check the backing plates for flexation before I pull the axles - assume I need the drums on to check.

The pedal and linkage all seem fine to me. Funny thing is, it doesn't seem to be so much about pressure as speed. Slow is good, slightly faster, same pressure (I think) isn't. I feel like it's all about the speed of that first inch or so of pedal.

Thanks again.

wyoming9

iTrader: (1)

Any up dates ?

Look at the Firewall for cracks but changing the booster I would think you would notice

mustrmrk

Thanks for asking. I swapped out the master cylinder today and there's a definite improvement. The pedal still gets hard if I press it too fast - and I think pressure does seem to affect it, too - slower = a lighter touch = solid brakes with great boost. But it's way less sensitive to that fast press than it used to be. That $50 Dorman MC was garbage.

I know I'm getting old, though, because during the swap I tested the check valve once again. Tested fine. After I got the MC installed, all I had was hard pedal. So I got out the old vacuum gauge and put it between the check valve and the booster. No freaking vacuum! The light dawned, I turned the check valve around, and, as they say, the rest was history.

So, bottom line, better, but not fixed, yet. It's going to be a few days before I get to check out the rear brakes. I'll definitely let you know what I find.

the_supernerd

I agree with Wyoming9, he always has good advice. I did think of a couple more things for you to check. Make sure that the rod between the brake booster and master cylinder is adjusted properly. I have never taken apart a brake booster to see exactly how it works, but maybe it needs a slight amount of resistance to work properly?

Another test might be to test one of the boosters outside of the brake system. Attach the vacuum line to it and push it by hand and see if it does the same thing.

Take it down a dirt road and slam the brakes enough to lock them up, then look at your tracks from the wheels to see if only one wheel or one axle is locking up. That might help tell you if one axle or wheel is binding at the shoes/pads.

Reman parts seem to have gotten progressively worse over the years, and I would really avoid reman brake parts if possible. The brakes on these RVs are scary enough when they work properly!

mustrmrk

Thanks for your suggestions, the_supernerd.

I just received my new booster actuator pin to master cylinder measuring tool today. I'd already used my calipers to set it, but didn't have high confidence in my methodology. So I bought the H-shaped tool - turns out I nailed the adjustment with my calipers, so I'm happy about that.

I also verified the pedal free play is within spec.

I seriously doubt I'll be able to get the rear wheels to lock up, even on a dirt road. I'll give it a try, but with no power assist when I slam on the brakes, I think I'll have a hard time - this little puppy weighs in at around 6500 lbs(4500 of which is on the rear wheels).

And you're right - even when the brakes are working well I drive very, very defensively.

the_supernerd

Since the new master cylinder helped the problem, it makes me think that excess play could be an issue. Make sure the rear brakes are properly adjusted when you get a chance to check them. Also make sure that nothing is leaking by peeling back the boots on the wheel cylinders/calipers. I think Toyota changed some 1-ton stuff in '87, so check that all of the parts are proper matching 1-ton parts. Someone might have got mismatched parts from the part store or something like that and managed to make them fit. Good luck!

wyoming9

iTrader: (1)

What has me curious is the difference between a Hard and slow brake application.

To me that says Mechanical rather then Hydraulic If kinked or crushed or partly blocked brakelines have been ruled out.

I wish I was close enough to help in person

I can never figure out Why the 22Re and Automatic was chosen for this Platform perhaps the Upfitter got the best price

mustrmrk

Yeah, funny thing. When I had the check valve turned the wrong way the pedal felt exactly like a fast press with the valve in the correct orientation. I thought I'd made it worse until I figured it out. What gets me is, even if one or two calipers or cylinders were binding, wouldn't I still have power assist on the others? Or is a kinked line or a binding caliper or cylinder enough to overcome vacuum assist?

wyoming9

iTrader: (1)

Without Lots of Algebra the best answer is maybe!!

the_supernerd

I feel like the problem might be the booster again, but I hate to tell you to get another one without verifying the problem first since this is already your second. Ideally you would take the rest of the brake system out of the equation. You could try attaching a brake pressure gauge directly to one of the ports on the master, then you would have resistance and could actually measure the pressure. You might also be able to gut an old master cylinder and put a heavy spring in it or something like that. Do you have access to a known good booster that you could try? Unless there are multiple problems in the rest of the braking system, I feel that you would still have boost to the brakes that work, and the working brakes would either lock up too easily or pull to one side. Can you feel the hard pedal/no boost when you are stopped or only when it takes extra pressure to stop?

Very true! I have seen the automatic RVs not be able to start out on steep hills, and O/D is absolutely useless unless you are going downhill. Swapping a 5 speed and 4.88s in a friend's RV was the difference between getting up the driveway or not.

wyoming9

iTrader: (1)

i like the idea of pressure testing off the master cylinder port with the one not used blocked if that shows a problem all well and good.

then the front brakes both sides

then the rear brakes you will need two gauges

Then you could block either port and do a test of front and rear brakes independently lots of brake bleeding in any case

To me it is the change in application between slow and steady and hard that changes

When do you say enough this thing stops good enough??

Since on brakes you don`t have a pressure relief valve to protect the pump when the systems is to the point where the pressure can no longer move the load for what ever reason the pedal will get hard

Then the age of the vehicle The quality of rebuilt parts or even new aftermarket parts.

Hang in there

mustrmrk

For sure. We have the automatic and 4.10 gears. The overdrive is very useful going downhill with a tailwind...

Wyoming and the_supernerd - thanks for sticking with me on this issue. You guys rock.

I'll see if I can beg, borrow, steal, or buy a brake pressure gauge somewhere around here.

Hopefully I'll have better luck doing that than finding a booster. The "proper" booster is not available. The only options I've found for an original fit are either send my old one to Rock Auto for rebuild (no cores available) or get one from MegaZip.com to the tune of $700+. There are no mid-80s one-ton Toyota trucks at any junkyard I've been able to find around here. So I went with a rebuilt dual diaphragm for an early 90s 4wd 4Runner. It's larger diameter than original, so i had to fab a 3/8 spacer for it to clear the steering shaft and adjust the linkage accordingly. When the brakes work, they work great. Given my understanding of how boosters work, intermittent, pressure-sensitive failure doesn't seem very likely to me. But what do I know?

On the one hand, I hope it's the booster so I can put this to bed. On the other hand, I hope it's something else so I won't have wasted all this time on a POS rebuild....

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