Brakes getting BAD! I suspect vacuum is related, need advice please!

[Rice Over Wheat]

I could use some help here as my brakes are getting progressively bad to the point that they are getting unpredictable stopping power at low speeds. It's kind of dangerous in rush hour commuting, but my daily driver.

I suspect this is vacuum related after 272 cams install. Before race cams I had great vacuum at idle and low speeds: -12 ~ -15psi. After cams I have -4 psi vaccuum at idle, but once I get to cruising speeds I sit at -10 psi and this is where the car performs great.

I've been having poor starting issues as I mentioned in another thread and I suspect poor vacuum to be the cause here as well. When the car is first started and I watch the boost gauge I notice a direct correlation between where vacuum is and whether the car wants to stall out on idle. When I give it throttle in neutral the vacuum falls to -10psi and the car runs great, but when I let off the gas the vaccum bleeds off to -3 to -4 and the car begins to stutter. Only after running for 40 secs or so does it stabilize and idle at 900. However, if I leave it idling for about 2 minutes it will do the same thing, vacuum will fall and the car will almost stall before the ecu kicks in to correct the situation.

Brakes -- there is an intermittant problem where the brakes have about 50% stopping power when at low speeds (0 ~ 20 mph), precisely when my vacuum levels are lowest. When brake pedal is pushed all the way in, the car takes about 4 feet to stop from a 10 mph roll.

Here's what I know from my own checks and that of a shop inspection:

-No vacuum leaks
-Brake fluid is full
-Brake pads at 60% thickness
-Rotors are ok with slight glazing
-All four calipers are functional
-Brake lines are intact, no leaks, no bulging, no air
-Brake booster checks out good
-Master brake cylinder does not leak
-Vacuum check valve is functional
-The faster the speed, the better the braking. Slower the worse it gets


Possible related issue: brake-torquing my fully built auto is extremely difficult after cams. It's tough to hold boost with brakes depressed. Brakes hold, just boost takes forever to build up compared to fast responsiveness before cams.

It is possible the master brake cylinder is going bad but I can't confirm this. If things keep declining then I will go ahead and replace it anyway to make sure.

Can poor vacuum cause poor braking at low speeds? Can it be related to poor idle? And lastly, what can be done to increase vacuum?

[Also posted to dsmlink, but dsmtuners tends to be better for non-link related issues]

 

[ROOKIE1]

My buddy has the same kind of problem . Different car for sure ? 70 Chevelle SS , @ 625 whp . He has low vacuum due to his cams . He was told he would need an electric brake booster . I also have BC 272s . My vac at idle is around 15-20 . I have not noticed any braking issues yet . I idle around 800 . Have you bled the brakes ? http://store.summitracing.com/egnsearch.asp?N=700+4294922533+115+4294910236&NeXID=5

 

[Rice Over Wheat]

I haven't bled the brakes because the master cylinder appears intact with no leaks. Since it's a closed system I don't see how air could get in there if the checkvalve in the vac line is working. I'll probably doublecheck this to verify, however.

One guy I know with BC 280s has about -9 ~ -10 vacuum at idle. As far as I know, I'm the only one with this issue.

 

[oldman]

I thought we've talked about your vacuum situation before, -4 inHG (-2psi) is definitely too low and you need to address that.

 

[Rice Over Wheat]

Do you mean on dsmlink forums? I had to fix my exhaust leak to see if that was skewing my O2 readings and run another log. Exhaust went in Friday so I'll do that tomorrow. I also wanted to install my wideband and verify my a/f, but fedex delayed it for 5 days for some reason.

I'd like to eliminate vacuum as the possible problem here but I'm not sure what can cause weak vacuum, particularly after installing cams, and I don't know how to troubleshoot the problem. I understand boost leaks and can track those down, but can you explain about vacuum?

 

[oldman]

Have you done a boost leak test yet? If so, what was the result?

 

[Rice Over Wheat]

No leaks until over 20+ psi at my meth injectors...I still need to teflon them. However, my boost is set at 20 psi so I don't see any boost leaks at where I am running now.

 

[oldman]

20 psi testing from the turbo inlet to the head?

 

[oldman]

Kevin, besides making sure that your 20psi boost leak test included the area between TB and head, go ahead and tripple check your timing marks and perform a compression test.

 

[Rice Over Wheat]

Yes the boost leak tester was in the turbo inlet and ran through the whole system, to the head. Ok, I'll verify the timing marks and compression tomorrow. Will update then, thanks.

 

[Rice Over Wheat]

Ok, after getting some help from a friend today, we've pretty much eliminated vacuum as a source of the braking problem. You can see the timing mark pics here if you have dsmlink:

http://www.dsmlink.com/forums/showthread.php?t=14047

Timing is perfect. We bled the brakes and there was a little air in the front passenger brake but after fixing this the brakes were still horrible. I can't even stop at a 5 mph roll without 3 feet of pavement UP hill! It's starting to look like I just need to upgrade my front brakes.

We isolated the vacuum problem though -- it was due to my meth injection kit. According to coolingmist's instructions for the Standard Trunkmount kit I ran a vac line from the intake manifold to the trunk...about 10 feet of line. We disco'd that and capped it off and the idle vacuum doubled to -10 psi and the car idled like normal 272s for once! After a couple pulls the boost responsiveness was very improved.

So the boost switch was moved up to the engine bay and reconnected to 4" of vac hose. But now the vacuum problem has pretty much returned to -4 psi at idle. He suspects my boost switch is faulty/leaking and I think he's right. I'm going to verify this at daylight by capping off the vac line to the boost switch again.

As for the brakes, I think it's time to upgrade the fronts.

 

[oldman]

Did you test the brakes at all when you had the vacuum at -10 inHG?

 

[Rice Over Wheat]

Yeah, same as in that above post. They SUCKED hardcore! I mean taking 3 feet to stop at 5 mph uphill? wtf

But, I took this as an opportunity (read: excuse) to upgrade and am getting the Baer Track Kit for the fronts. Not sure which to go with in the rear yet... Priority is fixing this asap though, it's dangerous!

 

[oldman]

Kevin, another thing you have not tested yet is the brake booster. When you pressure test your intake, you're not pressurizing the brake booster due to the check valve. Pressurize the booster after the check valve (not to much pressure on the compressor regulator) and see if the booster leaks. If it does, it can contribute to your braking problem and partly responsible for low vacuum. Realistically, you should be at around -14inHG.

 

[Rice Over Wheat]

I checked the brake booster already and it seems to be fine. I can pump the brake a few time and they will harden a bit, then start the car and the pedal will depress. If the brake booster wasn't working, the brake pedal should be very hard. It's soft if anything.

That vac problem I thought was isolated to the meth kit boost switch? I spoke too soon. The vacuum is still weak ast -4 or -5 at idle. I completely sealed the boost switch today so there can't be any leaks but I still have the poor idle. Back to the drawing board.

 

[moparmadd]

Have you tried machining the rotors yet? I mainly work on jeeps and sometimes we have problems with liberty's stopping. They will even do it sometimes with brand new rotors and pads. Not much fun when your going 60mph towards a lake on a test drive with both feet on the pedal.

 

[Rice Over Wheat]

My rotors are glazed...but I'm ordering a Baer kit tomorrow so I'll see if that solves the problem. If not, then it must be the master cyl.

 

[oldman]

Rice, do you still have stock brake lines?

 

[Rice Over Wheat]

Yes, but they appear to be ok. They'll be swapped to ss lines soon though.

 

[Rice Over Wheat]

RESOLVED!

Braking problem...was simple glazed rotors and pads. Upgraded to big brakes and braking restored.

Vacuum/idle problem...this was due to hacked 2G MAS. Never noticed a problem on stock cams. With 272s the car didn't like to idle and needed throttle to stay alive at start up and low rpms. Put on an unhacked MAS and it idles perfect, regardless of vacuum.

 

[oldman]

Never ever hack a 2G mas, glad you got it all worked out.