Woogies

So going on week two of having no truck because of a bad brake master cylinder. As luck would have it, I ordered the wrong part online. Apparently I need a master cylinder with a bore size of 7/8". The one I bought is a 1". So now I'm in a pickle, I need a car desperately or I literally can't get to work.

Will a 1" master cylinder work? Or will it not fit in the brake booster?

HighLux

What kind of truck? What year?
What was the part you got supposed to actually fit?

Woogies

Oh sorry. It's a 1994 toyota pickup (base, 2wd, no abs,Vin starts with '4'). I'm not sure, I read that the US made trucks used a 1" bore, the parts site said it would fit a DLX. So I assumed it was correct because I couldn't find any other info. And this why, boys and girls, you don't assume; the bore size is stamped on the old cylinder. 100% my fault.

No local parts places carry what I need either, that I found out last weekend... So I'm stuck

gennro

people swap 1" bore brake masters all the time. So yes it will work fine.

HighLux

Increased bore size wouldnt hurt. Does it physically bolt up?

guys upgrade to a bigger master when they put on bigger calipers.

you are just adding fluid volume and more pressure. I doubt 1/8 would be a deal breakerif it bolts up.

Woogies

That is the million dollar question. Which I will find out soon

nothingbetter

Many Toyotas use the same bolt pattern for the brake masters. Supra masters will fit as well. Plus you gain 30hp because its a supra part. The only thing you have to watch out for is if it has residual valves in it or not. My 1" from an fj80 has RV in both outlets but thats ok because I have rear discs. Put it on a try it, if it works, fist pump and drive on. If weird stuff happens, look into the residual valves. You may have to remove one.

HighLux

Nice catch. (tips hat)

Woogies

It work! yay!

However I noticed something. although 1000x better than having a bad MC, it still seems like I lack braking power. I have about 1-2in of pedal travel before it becomes firm and I can slam on them all day, but can't get them to lock up. Feels like I lack braking power.. Pads are fine, and bleeding was done well (pedal doesn't feel mushy anyways) and no leaks. Maybe the pedal needs adjusting? nature of the beast?

HighLux

you check all vacuum lines to booster? it wasnt doing it before was it? you bench bleed master?

Woogies

Master was bled. Well, considering I bought the truck with the mc on Its way out im not sure how it was before. I didn't fully test the lines but i glanced at them, nothing out of the ordinary

HighLux

Hmmm. It sounds kinda Booster....but we have a new part which is bigger, so we kinda dont know. I dont remember how to test the booster off hand.
But if it could be eliminated....thats a step closer.

If you pump the brakes up....will it hold full pedal then?

Woogies

Well in theory wouldn't a larger bore MC give me more braking power? I did a quick test of the booster (pump up with car off, hold with light pressure) doesn't appear to be any noticeable leak.

just to clear up, I can slow down quickly from around 35mph when the pedal is nearly down all the way, but I can't get it to lock up; which tells me i'm not getting the full braking power out of it and it certainly feels like it.

Does this truck have a load sensing proportioning valve? Did I need to bleed that as well?

RustBucket

iTrader: (1)

Using a bigger bore MC effectively reduced your braking leverage. Your pedal should get harder faster since it moved more volume. It will also take more force to lock up the brakes.

If you get a larger booster it will compensate for the additional force.

If you hate how it feels, look for a 7/8" or 13/16" bore MC. I have a used 15/16" I'd be happy to give you but that would only get you halfway to where you want to be.

Most of the time people upgrade to a larger MC when they get larger brake pistons. This keeps the proportion between the area of the MC and the activated pistons the same.

RustBucket

iTrader: (1)

And you likely have a proportioning valve, and yes you need to bleed it.

Woogies

Would that little size difference be that drastic of a change? I'm worried that it could be a safety issue..

Well I may have no choice but to deal with it if that's the case, which kinda sucks.. Now that it has been installed, I can guarantee I can't return it and boosters are about $150-$200. Granted, i talk about safety and ive been driving around with a nearly failed MC for two weeks; but I like doing thinigs correcty.....

arlindsay1992

iTrader: (8)

The area of the master cylinder piston is what determines the leverage ratio. Since area is related to diameter squared it makes quite a big difference with a small change in diameter.

As far as brake pedal free play, you may need to adjust the pushrod inside the brake booster. There is an adjustable rod with a lock nut that pushes on the master cylinder. It's located on the brake booster. It should be adjusted to a very small amount of play. I can't remember off hand but I believe the procedure and specs are listed in the FSM.

I'm not sure if the 2wd trucks use the same components as the 4wd trucks as far as brakes go. I know that the 4cyl 4x4 guys (myself included) swap in the V6 calipers, master cylinder, and dual diaphragm booster fairly often. The biggest change came after installing the dual diaphragm booster.

Woogies

Well that's just dandy. I might just have to live with it, because like I said, it's too late now to change it out and I just don't have the finances to make significant changes to the braking system simply because I put in the wrong MC.

RustBucket

iTrader: (1)

You might find that your 1" MC will fetch a good price on the board here, and smaller ones like 7/8" are sold very cheaply because everyone wants to upgrade.

osv

1" bore master cylinder is the correct part for upgrading the brakes, so you did fine there:
http://www.lowrangeoffroad.com/index...er/brakes.html

you just need to confirm what brake booster you have, and if necessary, get a used dual booster at the junkyard.

the dual boosters have a 1" or so step in the front side of the booster... Toyota boosters come in two different diameters, and either diameter can be a dual booster... it's a bit tricky, you want the large diameter dual(stepped) booster, I got mine off of a T100... and oh yeah, let me add that if you have a spacer plate behind the booster, leave it off when you install the dual booster.

the v6 calipers that are designed to work with the 1" master cylinder have two equally sized pistons... the smaller calipers don't have matching diameter pistons, which is why they use a smaller diameter master cylinder... it's all designed to work together.

you should be able to install it without disconnecting the master, just gently bend it(brake lines) back enough to swap the masters.

if nothing else, make sure that you have bled the brakes on all four corners, in the correct order.