MiataDoug

My Monster does not want to stop. Feels like I got no vacuum or no booster. I have the booster plugged into the rear outboard vacuum source on my explorer intake manifold. Anyone else have problems generating enough vacuum? I have a TFS1 cam. Does not just have weak brakes at idle. Does not want to stop going down the highway. the brakes are working, but I cant lock them up.

SupaDupaSteve

Do you know how much vacuum you've got? Lots of engines run vacuum pumps for that purpose (and others).

MiataDoug

I have 12 in-hg. I measured my stock Miata. It has 19 in-hg. I was just told by Jason that there is a check valve in the Miata line. I do not have that check valve installed. Maybe that is the problem?

.boB

Yes, that is the problem. Without a check valve, the booster will not work correctly.

MX-Brad

Yes, the backflow valve is in the hose the goes from the booster to the intake maniflold. Def need that. The valve is an inch or 2 from the booster.

MiataDoug

Well I found the check valve and installed it thinking for sure that would fix it, but it made no difference. maybe my booster is shot. There is a tiny bit of difference between having the vacuum hooked up and not. The brakes all seem to be working, just no boosting.

MiataDoug

one other thing. I pulled the vacuum hose off of the booster and there was vacuum on the booster an hour after last running the engine, so that check valve is doing its job and the booster is holding that vacuum and not leaking? But no boosting?

tbone heller

Got the valve facing the correct direction?

V8MiataMike

I think even if the booster was not working you should be able to mash the brake pedal hard enough to lock the brakes up. Are you using stock brakes? I would be looking for some kind of contamination on the pads (oil, etc) that have degraded the pads some how. Especially the front pads.

.boB

Take a look at how a brake booster works. It's really a pretty simple item. If it's holding vacuum, the booster is good. If the brakes are not working, look somewhere else for the problem. Could be something as simple as the brake pedal not fully returning.

MiataDoug

Thanks for the input. It does seem like the pads just are not biting. I can even smell hot brakes when trying to get them to lock up. I think I am going to take the calipers off and make sure they move freely and inspect/clean the pads and rotor surfaces with some garnet based sandpaper. I will check the pedal return. The rotors are drilled aftermarket units that I did not install. Probably not stock pads either. I have no idea how the car stopped before it was taken apart to install a V8.

charchri4

Sounds to me like a few hundred bucks in pads and rotors will solve the issue.

MiataDoug

I have not found anything wrong with the pads and rotors. I verified the pistons move smoothly and the 2 parts of the caliper that can slide do so. I pulled off the booster and hooked the vacuum port up to a stock Miata pulling 19 in-hg of vacuum and it works just fine under no load. (not installed). I am kind of fixated on the idea that my 5.0L is just not making enough vacuum to run the power brakes. It is only pulling 12 in-hg at idle. Does anyone know how much vacuum their power brakes are getting from their V8? Does some one have good power brake operation with 12 in-hg of vacuum? Thanks for helping.

V8MiataMike

Keep in mind that on typical stops the engine is in a deceleration mode creating much more vacuum than when just sitting at idle. That decel vacuum should "load" the booster so to speak when initially applying the brakes. I have a 67 Mustang I drive/race with a boosted 4 wheel disk brake setup (not much vacuum at all at idle - 7 to 8 at best due to cam, etc.) and it works good to get it slowed down on the decel after the run.

MiataDoug

Well my brakes are working. It was something going on at the booster, or with the vacuum source. I changed the booster and I plugged into a different vacuum source and I switched to an aftermarket check valve. I think it was probably the different vacuum source. Puzzling because I measured 12 in-hg at all of them. Now I stop great with no problems running 12 in-hg.