brake prob

[rhodesab]

just replaced my vac pump and have been trying the brakes out. problem- brakes stop well and pedal feels ok, second pump they are rock hard and have to put my whole body into the brakes to stop. any idea what the prob is Im thinking booster? truck is an 86 with 94 7.3 na cc with jerrdan roll back.

 

[LCAM-01XA]

Are you sure the vacuum pump is any good? Disconnect it from the vacuum tree on the firewall and hook up a gauge directly to it, see how much vacuum it pulls (should be at least 21") but more importantly how long it takes it to get there. If it checks out good, or tis a brand new pump (altho I'd still check it anyways), then it could very well be the booster - if the master cylinder has been replaced at least once chances are some of the brake fluid it leaked has found its way to the insides of the booster and has damaged one (or both) of the diaphragms. BUT it is also important to verify you don;t have any vacuum leaks elsewhere, for instance the cruise control has a coffee-can shaped reservoir on the driver-side inner fender that is well known to rust through at the very bottom...

 

[frank-id]

It would seem there is little or no vacuum storage. Start the engine for a few minutes. Stop engine and remove hose connected to tank on drivers inner fender. The brake booster could also be bad and uses all the stored vacuum quickly. Frank

 

[LCAM-01XA]

Frank, that canister is for the cruise control, brakes generally do not need a canister unless you're talking a gas motor with a nasty cam that pulls **** for vacuum.

 

[frank-id]

Alas, I disagree. The storage tank supplies vacuum for brakes, heating/Ac and some transmissions. Frank

 

[zukinut]

my bet is its the booster, my truck is doing the same thing and my problem is the booster.

Will

 

[rhodesab]

vac pump is areman from car quest/ ill hook agauge and check my cruise canister

 

[LCAM-01XA]

Alas, I disagree. The storage tank supplies vacuum for brakes, heating/Ac and some transmissions. Frank

We agree to disagree Just look at the size of the hoses tho, the booster main vacuum line is at least 3/8", whereas the canister uses 5/16" or 1/4" hose. Usually there's no need for a reserve canister, as the booster itself operates as one, even more so with the dual-diaphragm monster we have in our trucks. Additionally, there is a 1-way valve at the vacuum source port for the reserve canister, in case of vacuum loss in the main line that one closes and seals off the canister from the vacuum tree, so the booster cannot use any of that stored vacuum, and neither can the HVAC system for that matter - the cruise is the only thing that remains connected to the canister. I actually added a small vacuum canister especially for the HVAC, cause my vacuum pump is pretty weak so it don't always have enough pull to move the vacuum doors when I want it to.

 

[MIDNIGHT RIDER]

You are describing typical signs of a bad booster.

Once a booster gets bad enough, you won't even get the first application without the pedal pushing back against you, often accompanied by a loud "hisssssss".


That little ****** vacuum-canister has no bearing on your being able to apply the brakes without the pump running/working; the "canister" on vacuum-brake trailers is probably ten-gallon and good for maybe one application before it is exhausted.

You can replace the booster without having to dis-connect any brake-lines; but, if time and abilities allow, now would be a good time to convert to hydro-boost.




And by the way, do yourself a favor and install some vacuum-gauges, so you can better monitor what is going on.

 

[Agnem]

*sigh.

I'll post this again, probably the umpteenththousand time. If you don't have a vacuum gauge in your cab, right there with your EGT, Oil Presure, and Water Temp gauges, your going to be speculating on what your problem is anytime you have a vaccum problem. If you have a gauge, then you know how much vacuum you usually have at any given moment, and you know how long it takes to pump it down to that level. Once you reach the point this poster has, now it's just a parts swap fest as you guess what went wrong. The little storage can is needed only on vehicles with AC and vacuum cruise. Trucks with neither don't have the can, so the vacuum pump will run the brake booster by itself, with no problem. You could run a hose from the booster to the pump directly, to rule out all that other crap as the source of your vacuum leak, but at the end of the day, you still won't know if it is your pump or booster without that damn gauge.