bouis

I realize that a momentary idle drop is normal when braking, but if I mash the pedal quickly and repeatedly while idling I can nearly stall the engine. I'm not sure if this is normal on a first gen or not. Anybody?

Jeff20B

iTrader: (13)

Long Duck could probably answer this question. As for my opinion, I'd say it isn't normal. Probably a leak in the booster line or the booster itself.

bouis

I've checked the rubber hoses and they're fine. No cracks at all. I rotated the clamps and it didn't make any difference.

I can't hear any leaks and I don't have problems when not using the brakes. I don't really have any driving problems with it currently; it just doesn't seem right. I want to know if I should plan to replace it when/if I do a brake job or get a new rear end.

slashdawg00110

I've had this on most every manual tranny car I've driven. It's always driven me crazy. Booster looks OK; hoses look ok. I'm about to try clamping off the booster line to the manifold to see if that makes a difference, but I'm not sure I want to try braking without the boost. Maybe in a empty parking lot ...

bouis

Braking without the booster sucks, trust me. The first RX-7 I drove, was for a few miles, being pulled by a pickup truck with a rope (real classy, I know). Without the booster, the brakes were almost nonexistant.

MikeLMR

check the one way valve in the pipe, I also have this problem on one of my 323's ... doesn't stall but will stumble if you pump the brakes while stationary ... gives me somthing to do while sat in traffic if anything !!

kokujiro

I drove without the booster for awhile after I put in the new carb...all I can say is that my leg was incredibly strong after that experience. It can be done, but it's not fun.

bouis

You know, I don't think there is a check valve on mine. Should there be?

82transam

iTrader: (18)

Mine does this as well, as does my Trans am, I'm inclined to think its just one of those quirks or an old carbed car.

Sanspistons

Ditto on mine. Check valve? Hmm; I'll go check for one... Leaking boosters CAN be hard to diagnose and my engine idles too loud to check for hissing. Use a vacuum pump and see if it holds "vacuum".

Sanspistons for even better braking...

bouis

Does it hold vacuum? Good question. When I removed the hoses, it obviously relieved some vacuum. I wonder if I should try holding the pedal down for a minute or so before taking the hose off...

I have a vacuum gauge but no pump. I don't feel like buying one, either.

steve84GS TII

Whenever you push the brakes,you deplete the supply of vaccum in the booster.The booster is replenished by drawing vaccum from the engine.
What happens if you pull a large vaccum hose off the engine at idle?It dies,or runs really badly.Repeatedly pushing the brakes at idle is essencially like opening up a constant vaccum leak in the intake manifold,and it can stall the motor.EFI can account for the vacuum lose and keep the engine running.Carbs are just dumb pieces of metal with holes in them,they dont know there is a vacuum loss.
Thats just the way it works.

GavinJuice

That one way valve is usually the green plastic peice that connects to the brake booster. Take it out and blow on it with your mouth and see if it leaks.

Kentetsu

Might want to try this to check for a leaking booster:

Have somebody work the brake pedal while you spray ether around the booster. If it revs up, that means the ether is getting sucked into the intake via the booster.

Another trick that works well is take a piece of hose (in this case, size does not matter. lol) and put it to your ear. Move the other end around the possible vac leak and you will hear it loud and clear. I laughed when I first heard about this technique, but it works amazingly well. Good Luck!

LongDuck

Steve84GS TII nailed it right there.

This is perfectly normal for a car that draws the vacuum from the intake such as our cars. If you 'pump' the pedal enough, the vacuum loss at the intake plenum will cause idle inconsistencies, in extreme cases dropping the idle way down and stalling the engine.

ALL cars have a 1-way valve in the brake booster; more than likely it's INSIDE the brake booster feed hose and not visible from the outside. Mazda says to replace 1-way valve, replace hose. HTH,

bouis

Well, I'll check the hoses for that valve. Guys, I said that I knew it was expected; I just wanted to know if this degree was normal. Thanks.

Pele

iTrader: (28)

It's normal for it to stumble on repetitive brake steps, but I've never stalled a car doing it...

Try this:

Engine off, step on the brake pedal several times till it's rock solid... Press it down as hard as you can. Now start the car. Pedal should go down.

Turn the car off. Pump the brake pedal and the vacuum should be there for about three complete pumps till it comes up and is rock solid again...

Try leaving it overnight or even a few days... The vacuum should hold for a long time... If not it's leaking...

Try holding the brake down and listening under the dash with the engine idling... Can you hear a hissing suction noise? Give a quick shot of carb cleaner where the push rod for the brake pedal goes through the firewall... Did the engine rev any?

Take the check valve off the brake booster... Blow and suck through it at either side... Air should only go one way... (Remember to put it back the right way!)

Pele

iTrader: (28)

I can say that again.

Pele

iTrader: (28)

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Pele

iTrader: (28)

Pele

iTrader: (28)

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Pele

iTrader: (28)

GavinJuice

Just to add to that, your pedal should drop about an inch when you start your car.

bouis

I haven't been able to stall the engine, but it seems close -- sub 500 rpm. It does seem to hold vacuum for at least a day or so as evidenced when removing the hoses.

I read the FSM's section on the brake booster and I'm going to try all the tests, as well as the check valve, tomorrow. It doesn't pose a drivability problem, but I want to be thorough.

All_Terrain_RX7

I learned something new today. Thanks for the info guys. Now I understand why my car has gone dead on me after a near panic stop. I had the clutch in, really...