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View Full Version : Diagnosing a bad brake booster?


Mark Ashford
04-25-2007, 12:41 PM
After the last race, I was taking my nephews around the block for some joy rides and I all but lost the brakes... I can still stop the truck, but its a FAR and hard pedal.

Could be the MC, its still got fluid in it so I don't suspect a bad line or leak, but I thought I replaced the MC a couple years ago when I put the truck together.... but I might not have :D

I was also suspecting maybe I lost the booster, cause I know when you do it becomes VERY hard to hit the woh pedal when you loose the boost. So that got me wondering how you test the booster? Just pull a vacuum on it with a gauge and see if it leaks down? Or is there another (better) way?



Hmm, as I was typing here I wonder if maybe its the rear brakes.... They never really worked, I've never messed with them... but I would think the front brakes still would work just fine.



Thoughts, suggestions, anyone want to volunteer to come over and wash the truck so I can work on it :D


Opps, almost made the newbie mistake... Scout II, 304, 727 auto, I think the chassis was a '74 IIRC, the brake assembly was off my shelf.

thehud
04-25-2007, 01:09 PM
Well block off the vacume to the booster and see if it is the same on its braking (be sure to block off the vac line).

Also make sure that the booster is getting vacume from the one way check valve. That is how I would start.

Jim Grammer
04-25-2007, 01:54 PM
Mark, 2 simple diagnostics. 1)Engine off, pump the pedal a few times(no residual vacuum). Foot on brake pedal, start the truck. Pedal should drop. No drop, non-functional booster.

2)If you get pedal drop, there is vacuum to the booster. Then if you want you can apply vacuum with the hand pump and check for leaks in the diaphragm. I'd replace one that didn't hold vacuum.

Mark Ashford
04-25-2007, 02:49 PM
Thanks guys, definitely things I'll try.


Hud, ya, that thought occurred to me too right as I started this thread... unfortuantly the truck is at home, and I'm at work so it wasn't something I could try right way. I was hoping to also gather get a little arsenal of things to try if/when I get time this evening. Plus the truck is still on the trailer and if I go racing this Sunday, it would be nice not to unload it :D. Thanks for the reassurance that I was on the right track however.

Jim, as always, good info, hadn't thought of the 'pedal drop'.. and come to think of it, I don't think it was when I was last playing with it... definitely a quick and easy thing I can check on the trailer.

Any other thoughts from the peanut gallery?? I have this suspicion I'm replacing a booster.... now to just figure out 'new', or pull another one off my shelf.... both are tempting prospects..... ;)

Leeann
04-25-2007, 04:27 PM
We thought we had a bad brake booster on my '73 Dodge-based motorhome (had to practically stand on the pedal to stop). Even replaced the vacuum lines all over, but nothing helped.

In the meantime, I replaced the plugs, wires and coil. Voilá! Brake boost has returned. If you have a vacuum gauge, disconnect the line from the booster and put the vacuum gauge at that end. On the motorhome, it needed the gauge to read 20 before boost happened and before the tuneup it only read about 9.

scoutman800
04-25-2007, 04:32 PM
my brakes were never very good. replaced master with a bendix, calipers, wheel cylinders, etc.. ended up replacing the booster at one point and it made a big difference. someone on here posted that it can "test" okay and still be bad with the dual diaphram. wadda i know? GM makes a cool filter that fits inline to keep oil film and all outta yer booster. i grabbed one at AZ from the "help" section.

Paul "Misterfixit" Schulz
04-26-2007, 02:43 AM
my brakes were never very good. replaced master with a bendix, calipers, wheel cylinders, etc.. ended up replacing the booster at one point and it made a big difference. someone on here posted that it can "test" okay and still be bad with the dual diaphram. wadda i know? GM makes a cool filter that fits inline to keep oil film and all outta yer booster. i grabbed one at AZ from the "help" section.

Where exactly does this 'in line filter' go? i ask because the hose to the booster is connected to vacuum and has a one way flow valve in it at the booster which would prevent a backfire reaching the diaphram, so in theory no fluids should reach the booster that way, which leaves the only other and most common source of fluid entry to booster - brake fluid leaking out the rear of the master cylinder, standard non dot 5 spec (mineral) brake fluids will attack the boosters diaphrams.

Doc Stewart
04-26-2007, 09:44 AM
The one-way valve would not keep oil film from being sucked out of the intake manifold. Oil gets into the I manifold from the PCV when there is a little blow-by.

Mark Ashford
04-26-2007, 09:49 AM
Well, based on Jim's simple test, I condemned the Booster. No difference between engine on and off at the pedal. Plenty of vacuum at the port however if I pull the vacuum line (I didn't measure it however, good idea, I'll try to do that before I put it together).


As I suspected, the MC is 'new' from when I put the truck together 3-4 years ago, and I don't see any evidence of leakage were it goes into the Booster. The booster itself is quite crusty however. For grins I tried splitting it apart, but no go. Time has sealed it together pretty well ;)

So today I'll call around and see what I can find. I'm not opposed to a new MC/Booster combo... heck, I highly suspect the MC I put on this truck is a new one I originally bought for another project I'm going to try to get back on this summer... so a replacement would be good.


If worse comes to worse, I have another 'take out' sitting on the storage shelf :D


I tried putting a vacuum on it when it was sitting on the bench, but no luck. Its either torn, or with the MC off the front opening doesn't seal enough to make it work with the hand pump... o-well, this truck just might get manual brakes when it get 4-wheel discs... simplicity and less to fail on the course...


thanks for the suggestions, I'll keep you guys updated.

thehud
04-26-2007, 11:19 AM
For grins I tried splitting it apart, but no go. Time has sealed it together pretty well ;)




Be carefull!!!! Those tandem boosters if memory serves me are under a lot of spring pressure!!!

Eric VanBuren
04-26-2007, 11:22 AM
Mark, If you can't get it from Napa, Carquest stocks them at the Kent warehouse, thats where I bought mine.

scoutman800
04-26-2007, 12:38 PM
"this truck just might get manual brakes when it get 4-wheel discs... simplicity and less to fail on the course..."
i keep vasilating between full on hydroboost or manual. i like the idea that manual will feel the same all the time, engine on or off. with the 33"s on the 800 it took some arse to move the pedal, but it did stop and stop well (when the brakes weren't wet).

thehud
04-26-2007, 01:01 PM
i like the idea that manual will feel the same all the time, engine on or off. with the 33"s on the 800 it took some arse to move the pedal, but it did stop and stop well (when the brakes weren't wet).

I just can't get past the "pucker factor" when it comes to manual drum brakes!!! Will I stop.....or won't I stop!!!! I would keep a booster of some fashion if it were me.

Mark Ashford
04-26-2007, 01:37 PM
I just can't get past the "pucker factor" when it comes to manual drum brakes!!! Will I stop.....or won't I stop!!!! I would keep a booster of some fashion if it were me.


I can relate to that, I still remember when my trail Scout was 4-wheel drums, and only 32's... 'OK' for most things, but one 'oh crap' stop on the freeway when it was a LONG way slowing down was enough for those drums to GO AWAY...


Race Scout is disc however, and probably this 'off season' the rear (in op) drums will be going discs... then its the choice of boost or no boost. If I choice the MC well, the pedal feel should be good, and like was mentioned, won't change engine on, off, hing vacuum or non. Course power assist IS nice :D


Made some calls, new booster (booster only) is $161, Booster/MC combo is $210... or if I buy the Booster only and the MC out if their stock... the MC he stocks is only $16... do the math :D


However, since this is a LOW budget rig, and a off-road only rig... the FREE unit I have on the shelf if going on, I'll save the new parts for when I rebuild the thing this off-season.


Thanks for the help guys, I've never had a booster go bad, so this was a new one for me. We'll see what difference a 'new' junkyard booster does... or if worse comes to worse, I can pull the KNOWN good unit off my trail truck and swap it in, verify that is the issue (these boosters are way simple to swap :D)

scoutman800
04-26-2007, 02:46 PM
it's a wierd thing and i've seen it happen that someone will do brake work, push hard on the pedal first time and mess up the booster. i always push lightly at first till i have some pressure built up. not sure if it's an old diaphram that's ready to go anyway or what, but i had a tech do just that with an explorer that they were going to take off in for a trip to canada. unhappy customers that night! not suggesting you did that mark, just wanted to let folks know to go easy on the first pump or so. i'm anxious to see how your 4 wheel disc goes and whether you go manual or power. the 800 always stopped, but i do recall some whiteknuckle stuff going downhill right after coming out of a stream. it would squeal the 33"s, but you had to push hard.