Here is a picture of the old master brake cylinder that I just replaced on my 94 Montero:




I made the brake line tubes in order to do the proper "bench bleed" before installing the new master brake cylinder:




Got it installed, and bled all the brakes in correct order. When I hopped into the vehicle without the engine running, and tested the brakes, the pedal was nice and firm and only depressed partially as it should. However, after starting the engine the vacuum suction begins working in the Vacuum Booster (black round metal disc that the master brake cylinder mounts to). With the engine running, when I depress the brake pedal, it goes almost all the way to the floor board, and I have almost no brakes.

I once again bled the brakes, but it did not seem to have any air in the lines. Once again, the pedal is nice and firm without the engine running, and sinks all the way down with the engine running.

Can someone explain what is happening here? How does the engine vacuum going into the Booster cause failure of the brakes?
I thought the metal push rod went all the way through the Booster and made direct contact with the master brake cylinder, and that the Booster vacuum merely "assisted" in the process.

Thanks for all your ideas.
In the mean time, I will pull off the Booster vacuum line and plug it.....then I will start the engine and see if the brakes are working w/o the Booster vacuum.

 

Tip to others: Inside the rubber vaccuum line that goes to the Brake Booster, there is a check valve which allows the vacuum air to only go one way. I checked the hidden treasure in my vacuum line, and it was working fine.......only allows air to flow toward the engine, creating vacuum inside the booster.

There was good vacuum from engine when I started the engine and checked, so that is not an issue either.

The new master brake cylinder was bench bled, and then the entire brake line was bled (I used an electric vauum pump, which pulled the brake fluid out great).
However, I am getting no brakes, and the brake pedal just pumps fast to the floor, and does not pump up. I suppose its a bad booster?

These are the same symptoms the Monty had before I replaced the master brake cylinder.

Any other ideas before I pull everything apart and replace the booster?

Thanks

 

After further testing, it seems that the Vacuum Booster is fine:

When I hold my foot on the brake pedal and turn the key and start the engine, the brake pedal will go down as the engine vacuum enters the brake booster.....this is a sign of a good booster.

When I turn off the engine, and wait about 10 minutes, there is alot of vacuum pressure in the booster.......I slowly pull off the lower section of the rubber vacuum hose where it plugs into the booster, and there is alot of vacuum pressure being released.

When I remove the upper portion of this rubber vacuum hose, there is no vacuum release sound, which is a sign that the one-way valve inside the rubber hose is working as it should.

With the assumption that the booster was fine, I then felt that the ABS system was the problem. I did an "ABS Delete" today, and after bleeding the lines on all 4 wheels, I thought that surely the problem would be solved. Started the engine, and the brake pedal still goes to the floor.

Does this mean that the Booster is definately bad, even though it passed the earlier tests?
Pulling my hair out on this one.
Would appreciate any comments/help.

 

"Well I'll be dipped", as Vice Grip Garage would say.

Still baffled by this.

 

I'm really anxious to here the solution on this one. It's been years since I thought about the booster/system.
That is until a couple days ago. My neighbor has a bucket truck and experiencing soft brakes.
Bleeding the brakes without the engine running, quickly hardened the brake and obvious as air gurgled on the way out.
As soon as we started the engine the pedal got a bit soft as expected but didn't become as firm as one would expect after the initial boost vacuum set in.
What I heard shortly afterward was the "Whooshing" sound by the pedal each time I would press and release the brake.
I seem to remember that is a definite diaphragm issue but it's been 25 years since I dinked with them so I'm not confident.
All this to say appreciate you sharing this. Hope both of us get it solved.
Keep us posted! Thanks
jc

 

JC.....thanks for posting and showing interest in my brake issues.

I went ahead and tore everything out again so I could get to the master brake cylinder to remove it. Then I removed the Booster, and installed a new Booster. After putting all the parts/pieces back together, I bled all 4 brakes. While bleeding, the brake pedal does push fluid out of all 4 wheel calipers......so, I must assume that the master brake cylinder is working and pushing fluid through the lines. I then started the engine and tried the brakes.......NOTHING! The brake pedal just goes to the floor.

I am totally baffled as to what the problem is. I have put a new Master Brake Cylinder on, a new Vacuum Booster, and have removed the Anti lock brake system. The brake lines have been properly tied-in, and there are no leaks in the brake lines and no leaks at the 4 calipers. The master brake cylinder does not loose fluid as I keep pumping. The check valve in the vacuum rubber line is good, and the booster does get engine vacuum.

Today, I just gave up on it.......drove the 94 Montero out of my garage (with zero brakes and a hand brake that works a little). Just going to park it so I can tinker on another project. Maybe lightning will strike me and give me some insight as to what the friggin problem is.

I can present this one idea: The rear brakes have a "Proportioning Valve" that is designed to reduce the amount of braking (reduce the amount of brake fluid going to the rear calipers, based on the amount of cargo weight), and the proportioning valve has a return line that used to go to the ABS system under the hood. So, I guess that the proportioning valve rations off some of the brake fluid and sends it back to the ABS system. The ABS system had 2 input lines on the bottom.....one was from the Master Brake Cylinder, and the other was coming back from the rear Proportioning Valve. The top of the ABS system had 3 lines going out......one to the right front brake, one to the left front brake, and the other going back to the rear brakes (to the proportioning valve).

So, when I removed the ABS, I did the logical thing and routed the return line from the proportioning valve of the rear brakes back into the main Master Cylinder line that went to the ABS.......with the ABS deleted, I now have these two lines merging together into a block, and then the 3 brake lines going out. This was the design with the ABS, and should work fine without the ABS (in my opinion).

So, other than that, I am totally stumped.
Any suggestions from anyone???????????

 

Have you checked the flexible brake lines above each front caliper? With age these can fail in such a way that they "balloon" outwards. This can account for the issue only occurring when the booster is active, there simply isn't enough ooomph to accomplish this with foot pressure alone.

Edit: I'm not sure what setup you have on the rear, if you have drums back there, check the wheel cylinders for the same type of failure, the rubber boots on each end can do a very similar thing.

 

Naeos.......Thanks for the reply. Yes, the flexible brake lines on each of the front calipers are fine, and the rear calipers are good also. So, I have a new booster, a new Master brake cylinder, good brake lines....both metal and flexible, good one-way valve to booster, good vacuum in booster, and I removed the ABS system thinking that was the culprit. The brake pressure seems fine for bleeding all the brakes, and there are no leaks anyplace......but with the engine running, the brake pedal goes to the floor, and no brakes at all.

How is this possible?

 

Maybe a bit of a hassle but.... could one block lines/connections coming out of the master cylinder? (Divide system in half sorta)
If after starting, the brakes hold, you could be "relatively" confident the issue is the back half.
All that said it may not tell you enough but sometimes I need just a bit of assurance I'm at least "lost" in the right neighborhood.

jc

 

After consulting some of my buddies in the industry, they are saying you still have air in the lines somewhere. Try bleeding your brakes manually instead of using the electric contraption you bought, some of them have had bad results after using them. I went through hell trying to get a good pedal feel on my 98 MS but I eventually got mine where they work reliably.

I apologize if you already know the proper procedure for bleeding your brakes down manually but I wouldn't be doing you any justice by assuming. If you aren't familiar, watch this video. This video did me justice when I was just starting out and my "power bleeder" wouldn't do its job.