25 Master Cylinders (1 Viewer)

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Well I'm glad someone applied a little tech to this problem and actually measured the pistons! :) 1/4" sounds huge, mine must just barely clear the hole to be working now. I did check it from the reservoir with a wire and it was clearing the hole, which is all I cared at the time.

I have not noticed the same problem with the clutch, but it probably is happening. A little expansion will lock the brakes, but the same amount in the clutch would barely take up the slack in the release lever. I'm just happy the cylinders aren't leaking anymore, there's no way I'm taking them apart again.


Write a note to Marv, but I would sure hate to see the kit pulled from his inventory. ;p Anyone with a 25 should be checking in here for tech first anyhow. :flipoff2:
 
Yeah, I was surprised with that much piston length that I had any brake activity at all, even with it running cold. I guess I should've suspected something when I backed my pedal push rod all the way back and I still didn't have much free play.

Yep, I'll drop Marv a note to let him know . . . maybe he's just not aware of it or there's different variations on that particular rebuild kit or something. Who knows. :confused:
 
And, youi're proboably right about the clutch not being too problematic, especially with the ability to adjust the fork trension at the slave cylinder end.
 
Update . . .

Here's what we tried yesterday, and no brakes at all now:

Rebuilt, and rebuilt again, the already rebuilt MC using every possible combination of old and new parts (including both the new longer piston and the cleaned up original shorter piston).

Tried more brand new rubber cups.

Replaced retaining washer with a very thin one (about one third the thickness of the original) of the exact diameter and perfect fit.

Even tried putting the MC back together without a retaining washing . . . just the retaining clip, for even more rearward piston travel. Nothing.

Adjusted the shoes/drums again (and again).

Bench bled the MC each time and re-bled the entire system all around for a total of six times (and two tall bottles of brake fluid).

Tried every possible length of the pedal push rod.

Still no "good pedal." Can't even get the halfway down pedal stoppage that I had before. Just progessively more pedal resistance during the re-bleeds that eventually disappeared.

Shoes are activiating, but pedal still goes to the floor.

Any ideas????????

:confused: :confused: :confused:
 
Too short? Here's section of the cylinder, I dont think you can make it too short though. Even pumping it, you never get a hard pedal? How about the check valve, if that's not working fluid will just go in and out and you won't be able to "pump-up" the brakes. :confused:
brakea.jpg
 
Nope, can't even get a "good pedal" from pumping it. It gradually builds up some resistance after about 15-20 pumps, and then dissipates just as fast.

Ideas on verifying that the check valve is working right?

Is that pic from the 25's FSM?

Any chance you could post the rest of (or any more of) those pages you got there related to the Master Cylinder? I'd like to see the legend for all the parts listed and lettered, if that's possible.

Thanks :)
 
Also, Jim . . .

The above picture appears to show the end of the piston sitting just behind the larger rear hole with the cup sitting right underneath that hole. (Kinda hard to see in that pic for sure.)

Is that how far back the piston needs to retract? And, the cup is supposed to block that hole when it's in the retracted position, right??

Geez, this is getting frustrating. :mad:
 
FSM? :confused: Still waiting for one to drop from the sky......guess I'll have to start beating the bushes to find one. That picture is from the last edition of the parts book, eventually I'll scan the whole thing. ;)

M & N are the check valve, without it the shoe return springs just push the fluid right back when the piston returns and it won't pump up. If mine sits for a long time the pedal is low on the first pump and then right where it belongs (and stays there) on the second. You did say the shoes were adjusted, probably fine since they were dragging before.

I'll check mine tonight to see if the rear hole is blocked when the piston is fully returned.
 
Cruiser_Nerd said:
I'll check mine tonight to see if the rear hole is blocked when the piston is fully returned.
Click to expand...

Well, my rear hole is definitely blocked, even with the thinned out and/or no retaining washer. I'm suspecting that this quarter inch longer piston isn't the right one and just ain't going to shrink any shorter. :frown: So I'm going to try the original shorter piston again this weekend.

The 6 vent holes on the end of the old piston had one of the little cover wings slightly bent away from the hole, plus that start-shaped wing thing was pivoted off all six holes to begin with. Hopefully rectifying that might make the old piston workable again. Wish me luck. ;)
 
By the way, Jim . . . did I already ask you if you had measured and come up with the exact piston length that's required to make this darn MC work right?
 
Did I say tonight? I meant tonight.................not the other night. ;)

I haven't measured the piston length. I'm using the piston from the SOR kit.....I'll get you the thickness of the retaining washer, and check the two holes.
 
Steve,
I've got the SOR piston in mine. The retaining washer is sheetmetal .035" thick. When all the way back you can see the piston in the rear hole, not rubber. Here's some pictures but it's hard to see.
7250025.JPG
7250026.JPG
 
Thanks, Jim!!! I do appreciate that. :) I'm just wondering if the replacement SOR piston is the wrong one and way too long to begin with. A quarter inch is a lot. :confused:
 
I give up!!

Well, we've tried literally everything. :crybaby: :mad: :confused:

Different length pistons (even borrowed the piston and other internal parts from the working clutch master cylinder).

Different rubber.

New spring, old spring.

New check valve, old check valve.

New reatining wasnher, old retaining washer, fabricated skinny retaining washer, and even no retaining washer at all.

New retaining clip, old retaining clip.


Results:

No pedal of any sort.

No fluid going to the wheels (just sporatic major spurts of air during repetitive bleeding).

Seven empty brake fluid bottles.

And, even less hair on my head than I had before.


My mechanic buddies and I are all suspecting that this MC's cylinder walls have been bored out so many times over the years that it's had enough. As much as I want to stay OEM vintage, being able to stop this beast when it's moving is even more important.

So, I think it's time to sell out to the "dark side" for the time being and find a master cylinder that actually works. Any sugesstions for replacement (either single or dual circuit) that doesn't require a booster set-up??
 
My white one has been changed to a mid to late sixties single circuit and it doesn't require a booster. Might need to change the brake line at the master cylinder. I think the brake line is larger on 40s. I'm at work right now but I will check it when I get home. I don't know maybe the master cylinder ends would interchange but that would be to easy.

John
 
Tim Hill used to run dual circuit with front discs and no booster :). IMO if you're driving the thing at all, go dual circuit. Nothing more important than being able to stop.
 
Steve-

Did you try to pressure bleed the system? Not sure if your familiar with it, but pressure bleeding forces fluid through the system without pumping. It will get out air that you never thought was in there. If you want to try it, I can send you a tapped reservoir plug with an air fitting on it. :frown:

As for the dark side.......for a stocker I don't see a need for the power booster. If you're going through the trouble to change cylinders, get something with a dual circuit.
 
Does anyone have any specific model/stock numbers at any specific retail source for a dual circuit non-booster Toyota style MC?
 
No, I didn't try pressure-bleeding the system yet, but I don't think that's the problem right now. Even after we tried bench-bleeding the master cylinder numerous times, it still wasn't doing its job of sending the fluid out of it. But, thanks for the offer, Jim.

Don't get me wrong . . . I'm not going to completely sell out to the "dark side." :eek: I just want to get something to hold me over so I can drive it until I can eventually figure what's wrong with the OEM.

Does anybody have a link to one of these dual-circuit master cylinders that would work in the mean time? Do they even make them with the triangular 3-bolt pattern anymore?
 
picture of my 40 master cylinder

Steve here's a couple of pictures of my 40 master cylinder in my 25. If you look you can see the ends of brake and clutch cylinders look like the same size.

John
Img_0423.jpg
Img_0421.jpg
 
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