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Vacuum leak at Brake Booster?

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Norton47

Jedi Warrior
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Hi
I have been chasing a brake system issue, they work but seem to have different levels of firmness at different times.
The hoses have been replace with SS braided and the rear wheel cylinders and shoes and drums all replaced. The Master cylinder was replaced last year. I have bleed the brakes 3 times, and went through a QT of GT LMA, so they are flushed also.
Last night while checking to make sure the brake light and oil light were both dim and I had not tripped the PDWA in all my bleeding efforts, which I had not, I noticed that when I pumped the brake pedal at idle, the engine RPM's would increase 300 to 400 rpm. It will do this repeatedly.
Is this a good indication that the booster needs a rebuild?
If not what else is going on?
Regards
 
Actually, I believe that is normal. When you press on the pedal, the inside of the booster is vented to the air. Then when you release the pedal, it's exposed to manifold vacuum again. The effect is that of a vacuum leak, the extra air into the manifold lets the idle rpm rise.
 
Mine does not go up and down at all at idle if I pump the brake pedal. And if it did, I would think that 300-400 rpm would be excessive. Possible vacuum leak causing idle to go up that much?
 
I think Paul and Randal are both correct. The booster vents to outside so the vac is consumed and will bleed off some vacuum when repeatedly activated but 3 to 4 hundred RPMs seem awful excessive. My idle is about 800 and to go to 1200 seem very high but i don't remember repeatedly and rapidly pumping the brakes with the motor running. A lot of cars have a canister with a one way valve that banks a little vacuum to smooth out the use and to have a little available with the motor off, but my TR6 does not have this luxury system. Perhaps some one will be good enough to give it a try and see what happens to their Triumph.
 
Query, what do you mean by "pumped"? Fed design mandated that the vehicle had to have at least two backup pedal pumps when the engine was off, i.e. no vacuum being introduced into the reservoir. That's one of the reasons why the booster is so large, to act as a vacuum reservoir. Also, I have found that the plastic valve where the tubing fits into the booster will crack,deteriorate with age. So might be a good idea to check the valve and sealing o-ring for leaks,deterioration.

If you pressed the pedal just once or twice and got the rpm change you DO have a problem.
 
Ron
Substitute "actuating" or "depressing" the brake peddle 10 times or so in rapid succession. What RPM increase do you get?
 
Vette dog,

Reading the last post, I think you may have transferred Norton's(good bike, I had an AJS, ex sidecar bike once)post.

I have no problem with my brake booster and know how to diagnose it if one occurs. I was trying to relay a little information and knowledge that might help him diagnose his problem.

I've never had the need to do what you are describing to try to determine a brake booster failure. As the normal vacuum brake booster test is performed completely differently. Engine off, pump the pedal 5-6 times slowly between pumps, to let the vacuum bleed out of the booster reservoir, holding the pedal firmly to the floor, start the engine and ascertain the movement and resistance to determine the veracity of the booster.
 
Ok answers to queries.
I cannot detect any cracks or deterioration in the hose or fittings.
One, with engine at idle, any time the brake pedal is depressed the idle will increase then go back down. This is if once or 5 times in a row. Each actuation will cause the RPM to increase then with the pedal held down the rpm drop back down.

With engine off, pump 5-6 times and holding down, start engine.
Pedal moves slightly down, but not alot, but has downward movement. I would not say there was much resistance.
So RonMacPherson, what are your success criteria for your test?
Thanks for the comments
 
Okay, the booster is passing the industry standard test.

Another test to isolate if the vacuum is leaking out.

On the intake manifold where the booster line hooks up, remove the line and cap it. Fire the engine up, then repeat the idle speed changing test, see what happens, then move the vacuum capping down the line, i.e. put the rubber line back on the manifold, but cap it before the oneway valve. Then repeat.
 
Two small points. That funky vacuum outlet on the intake will easily get un-screwed by bumping it or the vacuum line. Making sure that it is firmly screwed down is important. Also, the plastic plug that seats into the booster should emit an audible sucking sound when pulled out of the booster, even days after the engine has been turned off.
 
There are two round red fiber gasket that go in front and behind the banjo fitting that Bill is speaking of. They are usually found to leak after cracking from old age and heat and loose fittings.
 
Update:
Fitting at manifold tight. Hose plugged when removed from booster check valve, engine idles fine, no change when brake depressed. Removed check valve, and detected that there was suction, thus I believe the booster itself was under vacuum. Check valve free and seems to work via lips to part and exhale inhale test method. Yummy.
Removed, and cleaned rubber gasket and greased with silicon grease and reinstalled.
Reran checks, vacuum seems to work fine.
Here's where I screwed up, I was not accurate in my description of number of brake pedal depressions. It really takes 3 or more quick depressions to make the idle change. Like you may do to pump up the brakes. I figure that is caused by the vacuum being quickly bleed off and causing a momentary leak as it refills (empties) the booster body. Tested by removing booster hose at vaccum source on the manifold and plugging the fiting, then with the engine running, crack the plug a little and the rpm rose and resealed and went back down.

So what is making my brakes have different degrees of firmness at random times?
I saw in a previous post that the master cylinder tipping valve could go bad even on a new part? Is there a way to check for this condition? My begining post has what has been replaced and the brake systems configuration is.
Any other suggestions?
 
Can you correlate the times they are softer with anything else ?

One known problem is the front pads being knocked back into the calipers, resulting in a low (not necessarily soft, but low) pedal on next use. Primarily caused by the front spindles flexing under spirited driving; it can be so severe that you have to pump the pedal to get brakes. But if your front wheel bearings are loose (poor adjustment or seats worn/distorted), you can get this problem even with normal driving.

Pardon me if I missed it; have you checked to see how long the booster holds vacuum with the engine off ? If it leaks down quickly, it might cause a firmer pedal (lack of power assist) if you've not had the engine on overrun for awhile.

One way to help isolate the problem might be to disconnect and plug the vacuum line to the booster and try driving it that way for awhile. You'll get no boost at all, of course, but if the pedal firmness still varies then you'll know it's a hydraulic or mechanical problem with the brakes, not a booster problem.

Have you checked that the pistons in the front calipers move OK ?
 
I will have to check all of the suggestions out.
Good ones ideas. I think the vacuum is holding but am checking this AM to see how well it has held overnight. I replaced the vacuum hose to the booster as it was slightly short and pulled on the check valve. In doing that I found that the manifold fitting did need new fiber washers and the other tap which was sealed off, the cap was deteriorated and leaking. Replaced that.
What is the second tap used for?
I have not had the caliper off yet. Will check bearing tightness. Was thinking of rebuilding those anyway.
Thanks
 
Brake saga update.
I have had the brakes really go down after a hard corner. Thought some how I had bleed off the vacuum. Perhaps that was spindle flex.
Brake booster holds some vacuum overnight. Verified by sound of sucking when check valve is moved enough to cause leak between it and seal.
Front wheel bearings checked. Right no detectable play at 12 and 6. Some play in steering when hands at 3 and 9.
Left some play detected when wiggled with at hands at 12 and 6 on wheels. Hands at 3 and 9 could not really tell do to slight play in steering. Seemed the same as right.
Tightened and remained about the same. I used the method of snugging up the nut and then turing back one flat to uncover hole for cotter key.
Ordered new bearings anyway and will replace both wheels sets.
Using a suction device, I bleed the system again following these steps.
I adjusted the rear brakes with the adjusters until they contacted the drums and sucked the fluid through. Made sure the reservoir stayed full. Did the fronts. Readjusted the rears untill the drums rotated freely, (I thought). Start engine and operate brakes. Engine off, pump brakes to bleed off vacuum, hold down and start engine and pedal travels down further once engine starts.
Road tested. Initially all felt improved, nice high pedal and consistant pedal throw or stroke. Then after a couple mile drive the rear drums were hot. Backed off adjuster and the pedal travels a little further but still firm. Continued drive. After couple of stop signs they seem to travel farther, but still brake, then a long overun and they seem nice and high, and firm. Travel some more and they change, as evidenced by more pedal travel.
Went home and went to bed.
Did feel good because the rear shocks I installed are nice. Won them on ebay, they look new and the oil was still clear and clean.
This morning I re- verified that the booster was under vacuum and disconnected and plugged the vacuum line to the booster.
The pedal travel is now consistant and of course takes more effort but feels right.
The left front locks up first, but not enough to swerve the car. It was raining this AM and I had the road to myself as I drove to work at 6 AM so I had a nice test track.

The rears will lock up and slide on gravel when I use the hand brake by itself this occurs when the handle is about 1/3 pulled up.
I could not get them to slide on the wet pavement. Slow down but not slide. Should they? This was at about 20 mph.

Maybe I am just to picky but the vacuum assist does not seem to be functioning correctly. I am wondering if it is developing an intermittant leak and sometimes can assist more than other times. Comments or suggestions
 
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