Weak Brake Power Assist

[jmgtp]

background info:
-91 GP/3.4 dohc
-Not an ABS car
-ceramic pads w/2k
-new rotors w/2k
-94+ rear brakes
-not a squishy pedal (no air in the lines)
-stops ok but requires abnormal amount of pedal effort
-6/6 cam timing (vacuum issue? brakes same as when -13*)

I have a modified intake manifold and I believe the fitting for the brake booster hose that I used is 5/8" diameter - what is the stock size? Perhaps the hose is too small? The hose is not 'squishing' under vacuum. I use a 1 way check valve to retain vacuum in the booster when manifold vacuum drops. I don't believe the booster is leaking. I have a hand operated vacuum pump/gauge that I can try to hook up to the booster and see if it maintains vacuum but given the size of the pump and volume of the booster I don't know if this would be a legit test.

What is the minimum amount of vacuum the motor should be pulling to have good power assist? I can throw the vac gauge on it and get the actual value the engine is pulling.

Could the booster be bad? How could I tell?

[55trucker]

One way to check the Booster is.....with the engine off, remove the vacuum hose to the booster nipple, then remove the nipple from the Booster itself. If the booster is still capable of holding a vacuum then the booster will suck in air as the nipple is removed seeing as the nipple is a one way valve. If either one of the two diaphragms inside have failed your pedal will be hard. Possibly one or both of the diaphragms have failed (no vacuum on either side of them) or perhaps the power piston has a failed seal.

[jmgtp]

Hey 55trucker that seems like a good way to test if it holds vacuum.

I always wondered if the nipple on the booster was a 1 way valve. When I swapped to the 96 intake there was a 1 way valve bolted to the intake manifold which I incorporated - so I guess now I have 2 one way valves... but does that mean the 96 booster did not have a 1 way on the booster itself? just curious on that.

Does the nipple just pull out? twist and pull?

On that note though, as soon as I shut the motor if the booster did not retain any vacuum I should not have that 1 final pump of the brakes after shutting the engine. I'm going to check that the next time I drive.

Sometimes when I shut the car off I do hear an air hiss... I always thought it to be the intake since I believe it started when I swapped over the the 96+ top end years and years ago... but maybe it is air rushing back into the booster and it is in fact leaking.

[55trucker]

I can't speak to the 96 booster assembly..but the nipple is a tight fit, you'll need to work it out. When the engine has been shut off you should under normal circumstances be able to *evac* the system by pumping down the vacuum in 2 or three brake pedal pumps, if that can't happen (the pedal is instantly hard) then I would say there is a vacuum leak somewhere (you might even hear it under the instrument panel from the brake pushrod).

[jmgtp]

Tried it today... I do retain 1 to 2 power assist pumps after shutting the engine, leading me to believe the brake booster has not been compromised.

Per my ecm via TunerProRT at idle the engine is pulling roughly 47kPa which works out to be about 14"Hg which I believe is not enough vacuum for full power assist. A quick googling turned up a lot of 17"Hg as the minimum.

So what do you guys with big cams do?

[pocket-rocket]

The pushrod guys swap out the springs from LSx lifters to basically turn stock lifters into Rhoads lifters, which don't give full lift until upper RPMs when the oil pressure raises to make them firmer. At least this is what I understand from what I've read. You could get a vacuum pump from a diesel car, or some Caddys used them too, from what I've read. Then just use it for your brake booster only.

[jmgtp]

I've thought about a vacuum pump before for this - it would be nice to have full braking power all the time. Problem is a lot of them look like they are run off the camshaft. I could probably fabricate a pulley on the end and mount it to be belt driven but it would be a lot of work. Plus I'd prolly end up spinning it well above its designed speed considering cams spin at 1/2 crank speed and diesels rev low to begin with. I wouldn't trust an electric one as my sole means of power assist.

I believe engines make the most vacuum at idle, but does the vacuum spike way up if you were decelerating (throttle closed) from say 3000 rpm? If thats the case, with the 1 way valve to retain vacuum I should have plenty to operate the booster...? Something tells me that my logic must be off.

Still brainstorming this one...

[55trucker]

Engine vacuum (most notably at idle) is a product of camshaft lobe design (leaning towards the intake valve and assuming that there is not a compression issue as well). Short duration/medium lift cams will produce more low end torque and along with that more vacuum. Increase the duration of the cam (especially on the intake) and your bottom end torque will suffer as well your engine vacuum. You mentioned that you did the 6-6 on the cam timing, I'm assuming that the vacuum problem became apparent after you did the timing change. Perhaps you might consider putting the intake timing back to OEM and see what happens to your engine vacuum after doing so ( Yeah..it will be a pain-in-the-ass to do). If this were 25+ odd years ago, and a vacuum gauge showed anything less than 25" (normal would be 30") a tech might be looking for bad cam timing (stretched chain), worn out cam, poor valves, lean carburetor, bad engine rings.

[jmgtp]

The low vacuum is evident with the 6/6 and the same degree as it was with the -13. I ran the -13 for years before and I honestly couldn't tell you what the brakes were like before that with stock cam timing. My vacuum reading is based off what the ecm was reading at idle rather than a mechanical gauge.

So, if the rpm was high and the throttle closed (decel) that wouldn't induce a greater vacuum than idle?

I don't have the time or energy to change the cam timing again - I've done so three times in the past month. Once when the heads went on, then again when I had to pull a head to replace a freeze plug, and then once more to change from -13 to 6/6.

I'm going to look at what some diesel cars use and see if it is something that could be adapted. I guess what I'm still not sure on is that I am not the only one with the 6/6 or -13, do others with these cam timings have low vacuum - at least low enough to impede the effectiveness of the brake booster? Or do I have some other kind of issue? The engine is no doubt old and has a lot of miles (18yrs/120k) but it still makes good power and doesn't burn oil, I'm assuming the rings are still good and the crosshatch on the cylinder walls still looked new.

One thing I noticed yesterday when I had the air cleaner off is that I have a lot of oil in the throttlebody/intake. A few teaspoons worth in the intake tube alone. It must be from the pcv system but the line from the valve cover to the TB (non vacuum side) should not be picking up oil since the airflow should be going into the valvecover. The line from the crankcase to the plenum is located low in the plenum chamber and I don't see how the oil would make it to the TB from there. Last year I ran a catch can on this line and it picked up a good amount of oil - I think I'll reinstall that and see what happens. I don't think this is impacting the brakes at all but it is a system that operates under vacuum so I thought I would mention it.