Power Brakes. Educate me.

[Eric.]

So my '87 MR2 has always had a weird issue with the brakes that I couldn't replicate, until I realized I wasn't doing it right. I lose pressure in the pedal whenever the car is at idle (i.e., in neutral or with the clutch pressed) and coming to a stop. I can pump the brakes to get pressure back for a moment before repeating it. I've just realized that the idling part is why its fading away, so if I downshift enough that I can keep the clutch engaged until I'm nearly stopped, it doesn't happen at all.

Please explain to me where my problem is.

 

[mPWRD]

So let me get this right: this is pre-ABS, and the brakes don't work while you have the clutch depressed. Also, do you lose brake pressure or pedal feel? They're two different things; i.e. does the car still slow down? Last question: you can get pressure back for a moment before repeating what?

From what I could glean, it looks like your problem resides in the lines or the rotors. Most likely the lines, but if you could clarify, I may be able to help.

 

[Punknoodle]

Vacuum leak to the master cylinder?

 

[Eric.]

So let me get this right: this is pre-ABS, and the brakes don't work while you have the clutch depressed. Also, do you lose brake pressure or pedal feel? They're two different things; i.e. does the car still slow down? Last question: you can get pressure back for a moment before repeating what?

From what I could glean, it looks like your problem resides in the lines or the rotors. Most likely the lines, but if you could clarify, I may be able to help.

Pre-ABS. They work momentarily with the clutch to the floor, but the pressure disappears in a pretty short time until you pump the pedal. Not feel, pressure. The brakes disappear. Isn't any sort of heat/brake fade problem, its totally fine during autocross and whatnot while the clutch is engaged and the revs are above idle.

Vacuum leak to the master cylinder?

Pretty much what I'm wanting to be told, I think.

 

[Perfect Balance]

Still having a hard time figuring out exactly whats going on. So the pedal stays where it is, and the car starts to roll because the brakes released? Or does your foot sink to the floor slowly?

If it's the latter, it's something I've noticed a few cars do over the year, and never understood why either. Usually on cars equipped with ABS though.

Might have something to do with the car being able to create more vacuum at idle, and therefore giving you more assist for the pedal, which makes it sink down more for the same given amount of foot pressure.

 

[Eric.]

The pedal sinks down but its losing brake pressure when it does. If I didn't let the pedal up and apply them again it would lose all braking power and would not stop.

I don't know anything about how vacuum works, I was assuming there was less vacuum at idle and that was the problem.

Edit: No leak in the brake system either, the level has always stayed full.

 

[Leonidae@MFT]

Sounds more and more like a vacuum leak. In a car that old it's not completely out of question.

 

[chinook77]

Sounds like a master cylinder fault. Internal leak where the fluid is bypassing back to the reservoir. Have you bled the lines to be certain no air got in?

 

[Zenith]

Sounds like unsuccessful bleeding or a vacuum leak. I'd a look around the vacuum assist for leaks. Braking has always been like this?

 

[niky]

Fluid. There's a leak.

 

[Perfect Balance]

Fluid. There's a leak.

Could be, definitely has the symptoms, but I've had plenty of cars that had this same weird phenomenon that didn't have any kind of fluid leaks.

Master cylinder could have an internal leak though, but that would be as simply as holding the brake pedal with the car off and seeing if it does the same thing, which I doubt.

 

[Keef]

The pedal sinks down but its losing brake pressure when it does. If I didn't let the pedal up and apply them again it would lose all braking power and would not stop.

I don't know anything about how vacuum works, I was assuming there was less vacuum at idle and that was the problem.

Edit: No leak in the brake system either, the level has always stayed full.

The brake booster creates boost from vacuum. Basically, there is a diaphragm inside - one side is exposed to vacuum, the other side is not. This lack of pressure on one side allows you to press the pedal against that side much easier. Because the throttle is closed at idle but the engine is still sucking air, there is a strong vacuum in the intake manifold, and therefore the whole vacuum system.

So let me get this straight. You're sitting still at idle. When you press the brakes, the pedal slowly sinks to the floor. Sounds to me like a fluid bypass inside your master cylinder. The piston inside is supposed to push on fluid, the seals keeping it in front of the piston, and therefore it compresses the fluid. If the seals are bad, fluid seeps behind the piston, and the pedal sinks as the piston moves forward to take the place of the fluid.

When you're moving, not only does the pedal sink, but clamping force also goes away. I don't know, that could be a result of the fluid bypass.

Either way, the sinking pedal is a symptom of fluid bypass, so I would suggest replacing the master cylinder. Unlike my car's brake booster problem, which apparently is very rare, masters go bad when they get old.

Don't even bother with a remanufactured part. Get a new one, pre-assembled with a new reservoir also. It's 90% guaranteed a remanufactured master will leak at the base of the old reservoir which you'll have to hammer into it. I tried two of them and both failed. Brand new one worked perfectly.

EDIT: Like PB said, get in the car with the engine off, press hard on the brake pedal, and see if it sinks. If it does, you've got bypass inside the master.

 

[Perfect Balance]

EDIT: Like PB said, get in the car with the engine off, press hard on the brake pedal, and see if it sinks. If it does, you've got bypass inside the master.

2 bucks says it doesn't sink to the floor with the car off.

 

[Keef]

What would cause excess vacuum?

 

[Eric.]

And the reason it works fine when not at idle if its a fluid leak (and remember, the fluid level has not changed one bit...)?

 

[niky]

What would cause excess vacuum?

Uh... clogged air fliter?

 

[Eric.]

Uh... clogged air fliter?

That's new!

 

[Keef]

Uh... clogged air fliter?

Engine would compensate anyway by opening the throttle more. The engine won't idle normally without normal vacuum, or vice versa. If any one of various sensors goes bad, something would be obviously wrong.

Master cylinder bypass is an internal leak. It goes past the seals and back into the reservoir.

I supposed the only way to check that while "not at idle" would be to hammer the gas and ride the brakes hard at the same time. Perfectly acceptable test technique. The only problem I noticed with this test procedure with the RX7, is that in first gear the front tires locked up and I pushed them down the road a few feet. Oops, lol.

While "not at idle" you wouldn't hardly notice a sinking pedal anyway, mainly because it won't sink as easily as there is less vacuum power assist, among all the sensations you feel as you slow down and you foot's habits of modulating the pedal.

 

[mPWRD]

So, in essence, you have a broken seal, or your lines aren't bled. Neither of which should be much of a problem to fix.

 

[Eric.]

I've driven down the road mashing the gas and riding the brake. That was how I was trying to replicate the issue before I realized it was only happening when I was coming to a stop with the clutch pressed or dropped into neutral.

 

[Punknoodle]

While "not at idle" you wouldn't hardly notice a sinking pedal anyway, mainly because it won't sink as easily as there is less vacuum power assist, among all the sensations you feel as you slow down and you foot's habits of modulating the pedal.

I don't really understand that logic, unless you are heel - toeing, wouldn't the engine be at the same if not more vacuum decelerating than at idle? This could explain why it's fine under deceleration just not idle, as the increased vacuum is making up for the leak. According to that logic though, when trying the brakes with the throttle open it should sink as much as it does at idle.

 

[Keef]

You're right - no matter the rpm, if the throttle is closed, there is strong manifold vacuum.

The way Eric should test for master cylinder bypass is to sit in the car with the engine off and press firmly on the pedal. If it sinks at any rate at all, fluid is bypassing the seals in the master. Do that and get back with us. In the mean time, bleed the fluid just because it can't hurt. If you're anything like me, you should save up and prepare to replace everything in the quest to solve a problem that nobody has ever heard of.

 

[mPWRD]

Or he could take it to a professional mechanic, describe the problem, and ask them about it.

 

[Perfect Balance]

Or he could take it to a professional mechanic, describe the problem, and ask them about it.

And they will drive it, not replicate the problem, return the car and charge him one hour whatever their labor rate is.

 

[Eric.]

Or they start replacing the entire brake system piece by piece, charging me for everything that isn't the cause, and ending up with a $600 repair bill for a new master cylinder.


I read up on how power brakes work...makes sense that it should be the master cylinder. Car is up on jack stands right now, I'll try a few things described here tomorrow.

 

[Eric.]

Yeah...so, engine off, pressing the brakes firmly...first time it sunk to the floor (not quickly) and after that it stayed firm and would not sink at all. Sounding like a vaccum problem again?

 

[Keef]

Yeah...so, engine off, pressing the brakes firmly...first time it sunk to the floor (not quickly) and after that it stayed firm and would not sink at all. Sounding like a vaccum problem again?

When you press it the first time you're using the last bit of vacuum in the booster, sort of like one flush after you turn your house's water off. When you let go of the pedal if fills with air and the pedal should get hard and stay hard. If it doesn't then you have a master cylinder bypass.

Apparently you don't have that problem, so that's good.

 

[Punknoodle]

Get some smoke in a can and look for vacuum leaks I'd say...

 

[MistaX]

I've driven down the road mashing the gas and riding the brake. That was how I was trying to replicate the issue before I realized it was only happening when I was coming to a stop with the clutch pressed or dropped into neutral.

This is so obviously a vacuum problem. It may not be a strong leak though... especially since you aren't noticing drivability issues. (Though they would be present, when you fix this problem the car will run differently, no doubt)


If you smoke, disconnect a vacuum line, take a drag, blow it out through the line, find where the smoke comes out. Done. It may be a good choice to blow it straight into the booster itself, to either confirm or eliminate that right off the bat. Though this problem could be caused by a vacuum leak pretty much anywhere.

If you don't smoke.... well I haven't figured out a trick for that.

 

[Punknoodle]

With a small leak you might need to blow bloody hard to get smoke to come out. If you were to use smoke in a can, you just spray it around the lines, booster, whatever you want to check, if there is a leak, smoke dissapears where the leak is. No need to disconnect lines, everyone happy and living longer due to not smoking