LJ78 Brake Master Cylinder and Booster Repair/Upgrade

[AirheadNut]

After sitting idle for a few months the brake fluid reservoir in my LJ78 was empty, with no evidence of an external leak. Bleeding the brakes restored some stopping power, but it is pretty lackluster. Given that it is still the original, 33 year old master cylinder I am planning on either rebuilding or replacing it, but wondered if I should consider replacing the booster at the same time.

With no external brake fluid loss, the only way I can see the reservoir emptying out is if the master cylinder leaked fluid past the seals and into the booster. Am I correct in assuming this? If so, is replacing the booster a must? Looking at part numbers, the original master cylinder (47201-60430) and booster (44610-60460) both appear to either be discontinued or simply not available. I have heard of people swapping to an 80-series master and booster setup, but am unsure what all is involved in that. If anyone could shed some light on what it would take to upgrade the master cylinder and booster and if the upgrade is worthwhile that would be greatly appreciated.

 

[Rigster]

It seems to be doable, and a thing. Especially if they have discontinued the parts.

Here is some info local here on mud... in case you did not find this thread already:

80 Series Brake Booster Upgrade

What is this 80 series brake upgrade, does it all bolt up without modification. Are there any benefits from changing to 80 series calipers too and likewise will they bolt up to 70 series hubs? My brake pedal keeps going to the floor, can bleed fine with engine off but soon as it's started pedal...

forum.ih8mud.com

There are so many prado's out there that I have to assume talking to a parts specialist you may find some parts are superseded to others in case you do not want to enter creative solution territory.

 

[AirheadNut]

Exactly what I was looking for, thanks @Rigster!

 

[AirheadNut]

Well, upon further consideration I decided to just get a master cylinder rebuild kit and go from there. Parts came in from @cruiseroutfit on Monday and this evening I pulled the original master cylinder out. Definite evidence of leaking past the seals to the booster, though whether fluid actually got into the booster or just leaked out the bottom I can't tell yet.

Inside the master cylinder there was a huge buildup of nasty gunk on the pistons:

Unfortunately while most of the bore looks great there is pitting along the bottom. I am going to try honing the cylinder to see if I can remove the pitting without taking too much out of the bore, but I am not hopeful. Looks like I may have to go the 80-series upgrade route after all.

 

[mt72]

I found a the BENDIX Brake boster from superior they claim it's 30% increase... I decided to give it a go and will report good job on the cleaning of the master cylinder! I will go that route soon as I feel the brake system need an extra push

 

[jblueridge]

The only thing I don’t like about the 80 series MC is the reservoir is at an angle.
I have to keep it nearly full to make the low fluid sensor happy.

 

[AirheadNut]

The only thing I don’t like about the 80 series MC is the reservoir is at an angle.
I have to keep it nearly full to make the low fluid sensor happy.

Good to know. Looking into all the parts necessary for the 80-series upgrade if that should become necessary, my main concern is with a deeper booster and brake lines on the opposite side of the MC I may run into clearance issues with my turbo.

 

[jblueridge]

I had my clutch booster and MC rebuilt by some folks here in VA.
Re-sleeving the MC is a disappearing art.
White Post Restorations – Fine Automobile Restorations & Brake Cylinder Solutions - https://whitepost.com/#hometop

 

[AirheadNut]

Spent some time with the brake cylinder hone this evening and it became apparent that the pitting is too deep to hone out.

Weighing the various options I think that going to the 80-series booster and master cylinder is the best route to take. I considered trying to make one of the other 70-series style MC's work, but that would require almost as much modification as the 80-series setup with no performance gains. Currently trying to hunt down an FJ80 booster that doesn't cost a small fortune!

 

[jblueridge]

I bought a rebuilt one from Rock Auto I think.

 

[AirheadNut]

Figured I should update this thread. I purchased a new aftermarket 80-series dual diaphragm booster, new Aisin 80-series MC and installed them with some stainless, banjo/-3AN lines and -3AN/10mm inverted flare adapters. Everything fit up nicely, the booster clevis rod was the perfect length to attach to the pedal without modification, the factory heat shield fit with a couple of minor tweaks, and the vacuum port on the booster was even in the original location.

I bled the master cylinder and braking system as per the instructions in the FSM, but braking doesn't seem to be all that great. The pedal is quite soft, stopping power is adequate but not great; certainly no better than the original setup before it started leaking internally. I've adjusted the booster pushrod, but probably need to bleed the system once more just to make sure and then readjust the LSPV. Pretty sure the front brakes are doing the bulk of the work.

One very odd thing I've had happen several times now is that all four brakes will occasionally stick on slightly while driving. I can just feel them catch when coasting to a stop, can smell them getting hot after a few minutes, and can feel the engine bog a bit and struggle in 4th and 5th. The pedal feel changes too: it will get significantly firmer when the brakes stick on. After a few minutes of driving they will unstick again. I've checked to make sure it is all four brakes; they were all quite hot when I stopped and felt the rims.

The last upgrade needed is an additional vacuum reservoir since the new booster uses a lot more vacuum than the original. A few hard pumps of the pedal in quick succession will drain the reservoir.

 

[jblueridge]

I had to bleed mine several times to get all the air out of it.
LSPV line that goes to the front might have been the root problem.

I put one of these on the right side down low.