80Truck

Had something "interesting" happen in the last week...

Usually I have good brakes, solid pedal feel, good braking. Twice now at different times I've had the pedal go to the floor and had minimal brakes. The second time it happened I unintentionally locked up the rear tires. Did a lot of down shifting to stop as the power brakes did not come back until after I began moving again... luckily neither time happened while in any kind of traffic...
then the brakes are fine...
Any ideas what it could be? I hope to have time this weekend to stick my head under the hood or where ever to sort it out...

TIA!

Terrys87

iTrader: (1)

Sounds like brake fluid is leaking past the seals in your master cylinder. A new Aisin Master Cylinder is not to expensive. You may need to find one on line but get an Aisin. Part stores are not the best replacement.

80Truck

I'm not sure that's going on. I can press firmly on the pedal and it holds and does not leak down. Also, the master was replaced by the PO. The booster was not and I am wondering if the problem is there. Thanks for the reply!

dbd6604

I too would be willing to bet that this is a bad brake master cylinder. I had this happen about a month ago on my Honda. EXACT same symptoms you're describing. I don't think a bad booster should cause the pedal to fall to the floor, but I could be wrong.

old87yota

Check for any leaks in the system first (a good time to inspect the entire system!)

This does sound like a master cylinder problem to me. The failing seal in the MC probably seals enough to brake until it hits some "gunk", then lets the brake fluid go right past the seal. My original master cylinder had a bunch of gunk in it which caused it to leak out the back.

A bad booster will NOT cause the pedal to sink to the floor, only a loss in hydraulic pressure like a bad master cylinder or a leak somewhere else in the system.

If you have a little time, order an Aisin master cylinder like Terry's87 said. It is worth it.

Here is my recent experience replacing my master cylinder about a week ago: https://www.yotatech.com/forums/f116...oblems-290711/

Good luck!

80Truck

Alright, I'll have to do more troubleshooting, and will keep all this in mind. Just a very different thing as I've had MC's go bad on other vehicles used to own nothing but AMC Jeeps and had the MC do the leaking down on an '80 J-10 pickup, Nothing like what this has done.

Would I be able to order the MC from a dealership? Or just find an online source, like Auto Parts Warehouse, or?

There are no leaks in the system, the current MC looks just like Old 87yota's, I just don't have a wire to plug the reservoir low switch into...
I can see about a picture or two maybe tomorrow if I think about it.

Thanks!

wyoming9

iTrader: (1)

Check Rock Auto but watch the shipping if it gets pulled from two different warehouses .

Go to Aisin Aftermarket look up the part number .

Search on Amazon or Ebay

80Truck

I can find all the usual brands on Rock Auto that I can get via the local autoparts stores at about the same prices... Did not see any Aisin brand parts....

I checked Aisin Aftermarket and they do not show anything for brakes, so did not find a P/N for Aisin...

I did do more with the truck, bled the brakes - at the MC and it worked correctly. Took a spin around the neighborhood with lots of stop signs and the brakes performed correctly... As stated before, there are no leaks in the system, all lines are rust free as well.

tj884Rdlx

Check for wetness around the lspv and connecting lines, just in case it is intermittent based on how it's loaded. But your bleed may have fixed it. Then again seals can be intermittent fail points too. Keep an eye on it - and increase your following distance for a while! Good luck

kawazx636

iTrader: (3)

If it is intermittent and you have no fluid under the truck then I'd swap the master cylinder. Not sure if you know how they work or not, but your entire brake system basically relies on a few soft rubber seals (SOFT - weren't not talking like the more rigid ones like main seals on a crank). Basically what is happening is the seals in your MC are getting worn to the point that sometimes they seal up well, other times they do not. The reason why it won't completely fail all at once and leave you with zero brakes is because the piston in the MC drives two different brake "circuits." In the case that your rear locked up, the seal in the MC driving the front brakes failed while the rear did not. It won't be long before they all fail though - let me know where you are so that I'll sure to stay away from you on the road...

Since you have no fluid leaks (I assume that is the case) then, the failure is internal. Bad seals in the MC are the only way you can lose brake pressure without leaking fluid. If the seals were leaking at the wheels then you'd be losing fluid and if the brake pedal was extremely firm then it would be a brake booster or vacuum issue.

80Truck

Thanks for all the advice folks.
The brakes were better after bleeding, but not to my satisfaction. I don't have time to dig around for an Aisin, so I picked one up from the nearest auto parts store and swapped it out, bled all the brakes and they are back to feeling correct. I'll keep looking for an Aisin to put on, but the one on it now will keep me rolling and stopping.
Dave, I'll take your info, from one shade tree mechanic to another, and file it in my manuals for the truck. And, no offence, but I avoid the police state of MD as much as possible, so no worries there.

Now on to the other thousand little things I want/need to do to this Truck!