Brake/Vacuum issue

[Loober]

Hi all,

I am at a loss trying to diagnose a braking issue on my 1hdft that I narrowed down to vacuum. Seems like I've tried everything....well except the thing that actually fixes it.

Heres the situation:

I am running a T100 booster and MC on my 77, have had this setup for a while with no issue until recently.

Recently started noticing the peddle getting hard after a few consecutive pumps, and losing braking power.

Things I've done:
replaced booster and master.
Replaced vac pump
installed a larger canister with a vac gauge
Replaced vac hoses to make sure no collapse is happening

What is happening:
Vacuum builds to 25inHg at idle and maintains
As I pump vac drops 5inHG with each pump
takes about 5 sec to build back up to 25 from zero
If I keep pumping it drops to zero and does start to build until I stop pumping.

I have never seen another system work with a gauge so I don't know what is a normal rate for vacuum to build.
All the cars I've driven that has never been an issue where vacuum gets used up after consecutive pumps, def not normal to have to wait for vac to build back up after pumping right?

Questions:
Should vac build instantly, does my 5 seconds seem right?

I've replaced the vac pump with another known good one, is there anything on the engine side that could effect its operation? Seem like the engine just turns the spline on the pump and the pump does everything. There are no weird noise coming from the pump when the engine is running so its not like the gear on the engine is slipping.

Any ideas of what else it could be?

 

[Loober]

test

 

[BreckenridgeCruiser]

Just thinking that you've replaced all the players in the system, but could a line be slowly leaking? Does it hold vacuum when the truck is shutdown and sits for a bit?

 

[BreckenridgeCruiser]

And is there anything else in the vacuum system? I was having a similar issue and it was the vacuum front drive leaking...

 

[BreckenridgeCruiser]

Just throwing out ideas as it seems like you've tried everything. Maybe a small leak in a hard line?

 

[Loober]

Yea I hear ya.
Tried pressurizing the system and listening for leaks, couldn't hear anything.
Tried running a hose from the pump to the canister bypassing all the hard lines, same.

I dont' know, its always something obvious and simple...

 

[BreckenridgeCruiser]

Crazy. I just looked up your truck (I think - 1991 HZJ77HV-MEU) and it looks like that truck came with a vacuum brake, clutch, and possibly front drive system.

Have you dug into the clutch booster system? It looks like there is a specific reservoir for just the clutch.

It also looks like there is a vacuum or electrical front drive option. Not sure if you have both the vacuum clutch and FD options, but you may try capping them off to isolate the braking system.

Hope that helps!

 

[Freewheel]

+2 on looking at the clutch booster, if you have one. When you hit the brakes, you generally hit the clutch as well, which can leak when engaged and kill your brake vacuum.
plug the clutch booster line to test.

 

[Jorgito]

Subscribing as I have the same issue with 1hd-fte, I did not have time to work on this issue so far.

 

[Loober]

Done the following since last post:

Tested Vac pump, by putting gauge at its outlet before going to the hard line, comes up to 27 ish inches immediately, and holds with engine off. Did the same chasing the rubber and hard lines up towards the booster, testing at every connection, all good. Once connected to the canister, take a little longer to get up to full vac, which makes sense because it has to evacuate the volume. Tried isolating everything else except the booster, including clutch, no change.

Another idea someone through out, is a blockage in the engine not allowing the vac pump to vent properly into the crankcase. Took the oil cap off to see if there was an issue with my catch can venting, no diff. So if there is a problem like that its between engine and vac pump. There is a small orfice on the pump that vents into the engine, can't imagine that getting clogged, its all sealed.

@Jorgito so your symptoms are the same? few pumps and the booster gets hard like it ran out of vacuum?

Its like the booster is using more vacuum then it should, watched a bunch of youtube video's on how booster's work...starting to think the rod is traveling too far, like I need to adjust the pedal and the rod where so there is less free play before it makes contact with the MC. Try that next.

But again, I've been running this as is prior without these issues...maybe it is engine related. Ruled the pump out as I've tried a spare and it acts exact same way.

 

[BreckenridgeCruiser]

@Loober I know you know your stuff and appreciate how methodically you are working through this. I know you'll figure it out!