Advice Please: 308 master cylinder/booster problem..

How do you test a master cylinder? I really doubt mine's bad, as it's brand new, but what characteristics does a bad MC exhibit? I'm just wondering if air is entering the system through it. The 2 line connections are snug. Wouldn't it leak if it wasn't airtight?

My guess is, I need to pull it out, and press the piston as I cover up the 2 circuit outlets. If the piston can't be moved as I cover the outlets, then it's airtight, right?

Does the junction where the MC bolts to the booster need to be airtight?
I would think not--the booster just pushes the piston, doesn't need to airtight, right?

Thanks, and don't miss my previous post!

Greg

 

Low resevoir = fluid likely in other 3 corners.

Did you bleed the proportioning valve as other suggested above?

 

Sean,
No, but I tapped on them and released lots of bubbles. The front one released air, the rear one didn't.

I guess I'll loosen the top fittings and see if I can get more air out.

You said "fluid likely in other 3 corners", you meant air, right?

Greg

 

Greg - you are correct concerning the mating of the master cylinder to the vacuum booster - an air seal is not important. It is a mechanical linkage that pushes the rod into the master cylinder.

Unfortunately, all bets are off concerning air in the other lines onces the reservoir sucked air. If the pedal goes to the floor, there is a great deal of air in the system. No other failure mode would lead to this symptom (other than a disconnect of the pedal linkage).

Even a faulty master cylinder would give resistance. The symptom of a faulty master cylinder is slow leakage with pressure on the pedal. The pedal, when pressed, will slowly go to the floor as fluid leaks around the seals in the master cylinder. One can pump the pedal several times to build up pressure, but when the foot is held firmly on the pedal, it will slowly find its way to the floor.

Looks like another bleed cycle of the 4 corners.

Jim S.

 

Jim,
You're great, thanks..

Yep, I'm gonna bleed all 4 again today..

If a master cylinder was bad, would it be a bit wet from fluid leakage?
Mine's absolutely dry..

I'm not able to get a high pedal at all, so it's gotta be air still

Many thanks, I'll report back.

Greg

 

Personally I think you are allowing air back into the system as you go. I would suggest a helper and use the jar filled with fluid and a short hose. Pump the pedal slowly to press the air out, then while the pedal is held to the floor by your helper, tighten the bleed screw. Then release the brake pedal. You should see steady improvement as you do each caliper.

 

One suggestion I don't recall seeing yet: Bleed the rears first. Longer lines and the balance adjuster in the mix. Dunno if it makes a difference with a dual master, but it's a standard I was taught long long ago.

 

Glassman,
Believe me, I've thought about this, but I'm monitoring the bleed line closely, no air is getting in that way, for sure. I keep the pressure on, as long as the bleed valve is open.

I'm beginning to suspect the MC is somehow allowing air in, but I don't see how. The reservoir is pressed on tightly, so the only other possibilities would be the line ports and mating flange, where maybe the inner seal on that side is sucking in air.. ? The line ports are tight and leak free, for sure.

DGS,
I have bled the rears first each time I've done this (about 5 X now... )

I'm gonna try again today..

Greg

 

OK,
I think I've narrowed down the problem.

I spent many hours today pushing all the air out of my system.

Went through several reservoir's full of brake fluid. That of course is a red flag--it shouldn't take that much fluid to push the air out--It took 4 reservoirs to get all the air out of the front left. Finally, the air was totally out.

Got in the car, started it, tested the brake pedal, after a few pushes it bottomed out. Opened my front left caliper bleeder, you guessed it, more air! I've got to be pulling new air in from somewhere!!!!!!

So, I called my neighbor to come over and push on the brake pedal while I held my ear to the master cylinder/reservoir area. I heard some sound that could be air hissing.

SO---- I noticed that my reservoir wasn't fully seated on the MC. Mind you, we're talking just an 1/8" or so, but I suspect the little barb, not being fully-seated on the MC is allowing air in as the MC upstrokes..

I can't think of any other source... My new question is:

I've pushed w/ all my might on that little plastic reservoir, but it won't fully seat. So, I drained it, pulled it off (was very tight, BTW..). My options are:

1) Remove the MC, push the reservoir on, off the car, then re-attach MC. This would be VERY difficult, because the 2 rear bolts would be pretty inaccesible.

2) Lube the 2 little plastic prongs and push it back on. Question is, what kind of lube is safe to apply to the prongs? I would just need a little, but it may get into the fluid......

Can anybody confirm that the reservoir must be FULLY-seated in order to prevent incoming air? Mine felt very snug, even though it wasn't fully-flush down to the MC..

Sorry for the lengthy post,

Greg

 

As glassman has stated already, use a helper.
Purchase a cheap bleeder(small cup with cap and magnet and hose $7.50). You can use a jar as we all did when we were teens, but it can be a little messy.
On my Testarossa I bleed front left first and so on. Verify res is full and thru-out stays at proper level.
Lower bleed valve first, then upper.
Have helper pump three times and hold. Use box wrench to open valve "ever-so-slightly" then close. Do this three times for each valve. Remember, have patience.
If this does not work, my advise is to seek an experienced F-car technician.
Chris.

 

PROBLEM - One man bleeding is difficult AND Bleed screw threads may let air in.

Use a speed bleeder at each caliper - this $6 device replaces your bleed screw on each caliper and looks nearly identical to the bleed screws but has a check valve in it. One man bleeding and no air entering from the bleed screw threads.
www.speedbleeder.com


PROBLEM - Bleeding brakes in wrong order causes line bubbles to flow to adjacent lines

Bleed the caliper furthest away from the Master Cylinder FIRST and progress to he next furthest from MC. This helps keep bubbles from flowing to the lines at the union points.

 

Synchro, according to WSM for my TR, brake bleeding starts nearest to MC. Now, if the WSM is incorrect, please tell me now.
Also, if you're associating this speedbleeder to me, I know nothing about a check valve. Just a clear plastic hose fits tightly around the nipple(love to say that word)and the other end to a small sealed cup with about an inch of clean fluid. Cup has magnet so it doesn't just hang there. One person pumps, one person to open and close valve. It's that simple. For an inexperienced home mechanic, I feel this is the least complicated method.
Chris.

 

Guys, I appreciate the responses. I can assure you, I've tried all methods.
I start from the rear, move forward. My MOTIVE pressure bleeder pushes very nicely, and it's worked before for me, w/ my OEM master cylinder.

There is no air getting in through my bleed valves. My calipers are new Wilwoods, and the bleed valve seats very tight. I just open it 1/2 turn, and the flow begins. I also tap the junction block, proportioning valve, caliper and MC with a rubber mallet to break loose more bubbles. It works. I'm using a long clear bleed line, going into a large plastic jug. NO air is creeping back into the line, I assure you.

On the one occasion I used an assistant to pump and hold the pedal, the bubbles never ended. He finally had to go home. The obvious reason is, the pedal pumping was sucking MORE NEW air in, probably through the MC.

Finally, when all the air is out, I press the brake pedal, and it goes all the way down. I then check for more air in the system, and THERE IS NEW AIR IN THE SYSTEM! Because I've done this several times now, I've experimented with different pedal pumps to see how much new air there is.
They correspond.

So, I can only conclude that my new MC is pulling in new air somewhere, but I sure can't see any leaks. I think the seal on the backside, where it mates to the booster, is bad. It pulls in air on the upstroke. My reservoir is fully-seated now, tight and leak-free. I STILL get new air when the pedal is pumped.

WHAT ELSE CAN IT BE??!! This is pretty much driving me crazy..

I'm gonna remove the MC and try to take it apart to inspect it. I bought it from Italy and I'm trying to reach the vendor, but I'd rather just get it fixed and use it.

 

It is counter-intuitive that a new part might be defective or not seal correctly, but since you're doing everything else right...

 

Scott, your instincts are right.

I just took it apart, looks great. All seals are tight, the thing just looks good.

I'm truly stumped as to where I'm pulling in new air. I didn't touch any connection other than the 2 lines out of the MC. It was great before I did this.
Feelin' pretty stupid right now....

I'm pretty sure it happens when I press the brake pedal. I know no air is coming in through the calipers. All connections are tight, everywhere. No fluid leaks anywhere.

Any thoughts would be appreciated. I've wasted 2 weeks on this wild goose chase!!

Greg
77 308, uprated Wilwood calipers/Coleman rotors/17" Speedlines...

 

After all the years I have spent around cars, it was quite a learning experience to spend over a year working on aircraft. The brakes on light airplanes are very simular, but actually simpler. Round o-ring seals in the calipers instead of square section ones used in cars. They dont use dust boots either. But bleeding them was quite interesting. I was taught by a guy who was 70 years old and who had been working on airplanes his whole life. He "pushed" the fluid up to the master from the caliper. Craziest darn thing, he used a hand pump can full of fluid, and simply put a shop rag over the bleeder screw hole, stuck the pump line in the hole over the rag (the rag created a seal) and pumped. All the air bubbles ran right up to the master and "viola", one man job, brakes working in a minute. Everytime. No muss, no fuss. I've tried it a few times on cars and havnt had any problems, yet.

 

Paul, very interesting.

What do you call these "push" pumps, and where can I get one?

Thanks,
Greg

 

OK, now I'm getting really puzzled!

I removed the new MC, put back the old one, hoping to isolate the problem.

I still have the same problem! I push all the air out with the pressure bleeder, get in the car, press the pedal, and it bottoms out. I check the calipers again, and there is AGAIN new air....

So, I believe pushing the pedal is INTRODUCING NEW AIR into the system! I took off the MOTIVE pressure bleeder, and had a buddy press the pedal as I bled each corner. Well, the bubbles never ended this way. I can only conclude that the pedal action is sucking in new air somewhere.

Again, there is NO VISUAL evidence of a leak anywhere. I've only removed the 4 bolts that hold the MC to the booster, and the 2 lines off the MC, nothing else.

Right now I should be back to where I started, because the OEM MC is back on, but I'm not...

Anybody have ANY idea how I can troubleshoot this further? The bad thing is, I can't even drive it to a shop, because I've got no brakes! I would have to tow it in.. That is definitely a last resort-- I know I can fix this...




Greg

 

With a dry master you will never get the brakes to bleed. You have to start by bleeding the master.



Brakes are really not a good place to start when learning to work on a car.

 

Thats how it is done on an assembly line.

 

Brian,
I appreciate your help, but this is not my first rodeo.

I've worked on brake systems before, on several cars. The 308 is the most basic of them all.

On the OEM MC, there is a bleed bolt on the side. I bled it first.

On the new one, there is no bleed bolt. Is there some other way to bleed it?

Maybe leave off one of the lines, and run fluid through it?

Greg

 

The bolt is not for bleeding. That should never be removed.

No offense but your last comments and lack of understanding of the problem you are in and how to get out of it are all I need to know you are in over your head.


Your situation was literally covered in my first week on the job.

Leave brakes to someone that knows how they work.

Others lives count on it.

 

The MityVac Silverline pumps can be used with the MityVac brake bleedint accessory to pump pressure as well as suck.

OR,

you could use a reducing barb fitting to adapt your Motive pressure bleeder's ~3/8" hose to a small (3/16 I think) hose that's a tight fit on the bleed screw.

BTW,
Those are big calipers, I can see it taking a surprisingly long time to get all the trapped air out & force the pistons firmly against the pads.

TIP:
Put a small amount of Permatex Hylomar, or plain bearing grease on the bleed screw threads to help them seal.

Here's something to try:
Use a board in front of the seat to keep the brake pedal about 1/2-3/4 way down.

This positions the master cylinder's piston so that fluid (and air) can't back up into the reservior.

Then use a bleed screw to pressurize the system using a MityVac or your Motive pressure bleeder.

The system sholud hold pressure indefinitely. If it doesn't go looking for the leak. If you're pressurizing with fluid, it should produce a fluid leak. If so, go looking for it.

If the MC is bad, you won't be able to pressurize the system as the MC will be letting fluid back up into the reservoir.

 

Greg, I just installed a new MC in my TR.
After connecting everything and filling with clean fluid, I had my seven year old son push the brake pedal a few times and hold. I then loosened the brake line on the side of the MC just barely to expell any air then tighten(use plastic under MC).
Then did the other one. I did each maybe three times. I think I used 11mm wrench.
Then came the wheels and clutch.
If it weren't for the under-dash nut that holds the booster to the firewall, you know, the one that Ferrari used midgets to install at the factory, It would have taken me thirty minutes tops.
Just try this method.
Chris.

 

Brian,
FYI, I was able to get a bit of air out of that bolt hole. Of course, I had to depress the brake pedal to get it back in.

I would never drive a car w/ no brakes. No lives are in jeopardy, don't worry!

Verell,
Thanks so much for the phone assistance!

Chris,

Not a bad idea, I'll try that!

Greg