Long explination, sorry, but Ive been fighting this problem...

summary: recently my car has been feeling like there's no vacum assist when braking - symptoms : very hard pedal, takes ALOT of effort to slow the car down.

however if I put ALOT of pressure on the pedal I can still get the brakes to lock up the tires - so it seems to me that the system is still hydraulically sound.

here's a clue though: if i let the car idle, or if I drive for 30-60 seconds without applying the brakes, it seems to build up enough vacum to provide ONE assisted stop, the second stop is very stiff and requires excessive effort. - also, if I do a few short, quick taps of the pedal it gets harder and slows the car down less with each succesive tap.

-what ive done to fix it: I've verified that the motor pulls 15in/hg of vacumm at idle-
I verified this at the barb above the 6port actuators, AND on the short hose that connects that to the hardline along the firewall, AND on the drivers side of the hardline, AND on the end of the hose that connects the hardline to the booster.

I tested the Booster for vacum leaks, and it holds vacum rock solid.

I'm also using a NEW MAZDA brake booster hose with the valve preinstalled at the factory.

Doing all this actually SOLVED my bouncing idle, it's a dead solid 850rpm - so that makes me think I cured a vacum leak somewhere, but still, my brakes no worky - ANY ideas?

 

check for vacuum leaks then come back and report your findings.

 

as I said. I checked for vacum at all brake booster hardlines, all hoses, and at the booster, all 15 in/hg

 

I think I Know the problem here,,Go to the BOOSTER and take the Rubber hose that goes from the Booster and leads to that steel Line that runs across the Firewall.Remove the clamps..Reverse,The hose(FLIP IT over),and then install it again..That hose has the ONE WAY Chcek valve in it..It Sounds to be that the hose is just Not Installed properly..so you are NOT getting Vaccum assist from the Engine,The brakes feel Ok,But you have to Literally Grab the steering wheel and Pull your Whole Body onto the brake pedal to stop the Car!, So it is Installed the WRONG WAY.(remember is is a ONE Way Valve)..I did this mistake,while changing Brake boosters..It Stumped me..Until I had an Epiphany in My Sleep!..Thank god I work on my Car in MY dreams!..good Luck man..Send me a PM if this happened to be the cause..I would like to know If Indeed that was the culprit!...STYX.

 

Pull the hose that runs from the exhaust side of the engine (rear rotor area) to the metal tube that runs across the firewall to the brake booster.
I had the same symptoms. My hose was leaking (old, cracked). Replacing it solved the problem.

 

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Yup that the same one that got me also. I looked fine but when you bend that molded hose backwards all the cracks were exposed.

 

Is it possible that the check valve is in backwards?

The brake booster has a check valve that is supposed to avoid the loss of vacuum in the booster when the throttle is floored. If this were backwards, it would only leak air out of the booster slowly

It's a long shot, but I have no other ideas

 

Ok, so I eliminated a few possible causes:
-I replaced the hose on the manifold side that runs to the firewall hardline with hi temp silicone hose
-I double checked the new check-valve hose on the drivers side, it's instllalled correctly - allows free flow of air out of the booster, but no airflow into it

how about some theories from people who know the inner working of the booster itself, what does it look like in there? I know it holds vacum if I attatch a line to it and pump it up to 20in/hg - what about parts that might fail internally on the booster? any ideas?

 

Oh and for what its worth, Right now I have the ORIGINAL booster installed,
I've already tried a new booster (which was my first attempt at solving the problem) but that didnt solve things, if anything, it's a LITTLE better with the original booster.

could I have gotten a BAD new booster? how can i test it?

 

Hard to believe you lucked out and got another bad booster. I didn't really think the check valve was in backwards because it worked prior to your *problem*. But I had that jpg laying around and posted it.

Sorry, I've not a clue what the problem is. If the hose are good and you tried another booster, what's left. Another booster? Unlikely.

Are you absolutly sure the hose is connected up to the correct nipple on the back of the intake manifold? Not somehow mixed up with the split air pipe's connection?

I probably shouldn't have posted anything. Oh well.

EDIT: Have you ever connected up to the line that connects directly to the booster and read the vacuum there? Just the booster out of the picture, nothing else out of the picture.

Have the brakes worked right in the recent past? This is just something that went bad recently?

The FSM BRAKES section shows the guts of the booster and how to put it together/take apart. Still hard to believe two boosters bad.

 

Which wheels lock up when you push on the pedal hard? When a master cylinder fails it will have those symptoms. If only certain wheels lock up then one of the circuits has failed.

 

This is not a hydraulic issure, definately a vacum issue,

Can anyone post better pictures of the fsm where is shows how to hook up the various pressure/vacum gauges to test the booster? My copy's illustration is impossible to see. even when zoomed.

I also looked at the diagram of the internals of the booster, could the rubber diaphragm on both my old and NEW boosters be ripped?

-the way I understand it is that the "boost" in braking comes from a pressure differential on either side of the diaphragm inside the booster ie: 1 bar of pressure on the rear side of the seal, and hopefully nothing but vacum on the other side, that way the vacum will help you push the pedal down.

but IF the diaphragm had a leak, it wouldnt KEEP vacum on the CORRECT side of the booster chamber, and thus not assist braking, however if the BOOSTER SHELL was still airtight, it WOULD still hold vacum from the outside on a vacum test, but only because you'd be creating vacum on BOTH sides of the diaphragm.

I guess at this point I may be down to returning the "new" booster and getting another "new" one

unless anyone else has a better idea? .......

 

sounds like 2 bad boosters to me... i went through 3 at one point on my old chevy.. all tested good until i hit the brake...

 

I'd still like to see a better copy of the picture in the fsm that shows where to hook up the various gauges in order to test the booster, anyone have that?

it's the first ilustration on page 11 "power brake unit"

But I guess I'll take the booster back and get another one, chich means anoer week or two with out power brakes, oh the joy...

 

my fsm also looks like ****, we must shop at the same store lol

 

I don't think Hailers pic will show in my post ,,BUT.
See that hose that Hailers is Showing you ..That one..take it and FLIP it..180 degrees..That hose is the one that has teh One Way Check valve in it(I assume you know that.)..but if it is on WRONG,then you Don't get Vacuum...another Poster mentioned the Hard line,,I would see if that line is Perforated and letting air in(leaking) also.

 

hmmm, I'll plug one end of the hardline and try a vacum test on it

 

tested the hardline, no leaks there.....

 

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thanks hailers