[T2] Vacuum assist

[Bob Pratt]

Hi Jamie,

As I was writing that, I was confusing myself.
 Is the adjustment in front of the master cylinder? How does one adjust it correctly?
It was suggested to me that the check valve might be bad. I removed it today and tested it. It was fine. So I'll send the new check valve back when it comes in.
Bentley said that no adjustment would be needed if I was just changing the master cylinder. 
I'm confused and frustrated!


Bob Pratt

> On Aug 31, 2015, at 10:06 PM, Jamie <jrivers at globalserve.net> wrote:
> 
> maybe it is just semantics or typos,, but i am not clear on a few things..
> 
> ______point_1__________________
> you said...
> "The bus 1971. According to Bentley, the adjustment is on the front side of the vacuum assist as it doesn't have the short adjustment rod like the pre-1972 busses have."
> 
> I don't understand what your wrote..........  a pre 72 is a 71 ??  ,,, did you mean pre 71 ?
> 
> any year bus with disc brakes on the front should have a booster..... but not all........
> so,, usually a 71 and newer has disc brakes on the front.. so a71 should have a booster
> 
> your 71 has a booster......
> 
> the rod adjustment is critical...... on the booster style... there is a 0.040 clearance required... that adjustable rod is part of the booster...
> if i remember correctly,,  you have to "feel" the clearance .. as you can't see or measure it.  elaborated a bit more below...
> 
> ______point__2__________________
> 
> you caid
> "I disconnected the brake pedal from the shaft that goes into the front of the vacuum assist and there was no movement of the pedal. "
> 
> I don't understand that ...... if the brake pedal is disconnected from the shaft.. the pedal should have lots of movement.
> 
> I think you are trying to ensure there is clearance 0.040 thou... which means you remove the master cyl.. and measure the depth of the hole.. and measure the length of the rod.. and adjust the rod to be 40thou shorter..
> or
> you "feel" that 40thou by adj the rod tooo long,, put the master in place without the O ring .. and measure the gap.. then shorten the rod... until there is bearly a gap,, then shorten it 40thou more..
> 
> ______point___3_________________
> 
> you said
> "I also noticed that the vacuum hose from the vacuum assist filter had no clamp on it wand was just sitting on the connection."
> 
> that hose to the filter doesn't need to be there to make the brakes work... it is only there to ensure clean atmospheric air pressure makes it to the booster..
> if it is a tight push fit,, it is ok.. won't hurt to have a clamp,, but it doesn't need one..
> it isn't a vacuum hose.. as that is not the vacuum side of the booster.. it is a regular hose to allow atmospheric pressure,, make sure the end of it is tucked into the body . or somewhere that dirt isn't getting into it .. sometimes it sits in the wheel well where very dirty air can enter the hose.
> 
> ______point__4_________________
> 
> you said
> "I should be getting the vacuum check valve on September 5th."
> 
> Bentley says the check valve is only there to prevent an engine backfire from putting pressure into the booster...
> other sources say the check valve keeps vac in place while the throttle is open and the engine is producing near zero vac at that point.. so the vac that is trapped inside the booster will still allow a couple of brake applications before the assist is not longer working.. and the pedal becomes a manual brake system
> 
> 
> 
> 
> ----- Original Message ----- From: "Bob Pratt" <ka1dza at verizon.net>
> To: "Jamie" <jrivers at globalserve.net>
> Cc: "Old Volks Home" <oldvolkshome at gmail.com>; "Paul" <saulsbury at cox.net>; "Type 2 List" <type2 at type2.com>
> Sent: Monday, August 31, 2015 8:59 PM
> Subject: Re: [T2] Vacuum assist
> 
> 
> The bus 1971. According to Bentley, the adjustment is on the front side of the vacuum assist as it doesn't have the short adjustment rod like the pre-1972 busses have.
> 
> I disconnected the brake pedal from the shaft that goes into the front of the vacuum assist and there was no movement of the pedal.
> 
> I also noticed that the vacuum hose from the vacuum assist filter had no clamp on it wand was just sitting on the connection. I put a clamp on it, but it made no difference.
> 
> I should be getting the vacuum check valve on September 5th.
> 
> Bob Pratt
> 
>> On Aug 30, 2015, at 3:51 PM, Jamie <jrivers at globalserve.net> wrote:
>> 
>> i dunno.. the way I understand the valves in the braking system,, the port valve inside the booster causes the diaphram to assist the movement of the application rod
>> once you cover that port ,, the vac removes the air on one side of the diaphram ,, and allows atmospheric pressure to assist in applying the movement to the brake pedal
>> 
>> so,, if the master cyl wasn't installed with the correct clearance,, and it is already in a state of application.. it will be difficult to impossible to bleed,, and it may cover that port valve already,, so as soon as the vac is applied,, the pedal will move toward brake application.
>> 
>> OR
>> if the old master cyl leaked fluid into the booster,, there will be fluid in the booster...
>> once there is enough fluid,, that fluid covers the port valve ,, the vac starts to be unequal on each side of the diaphram , and atm pushes the pedal toward the master .. applying the brakes and it will appear all on its own.. if you turn off the vac [engine] the spring in the booster will return the brake pedal to normal position
>> 
>> the check valve holds vac in the booster while there is little or no vac coming from the engine,, during open throttle situation ,, the check valve doesn't apply the brakes, it will allow equal vac on both sides of the booster diaphram.. ... or if installed wrong.. no vac to the booster .. or if it isn't present or malfunctions so it breathes both ways.. you will have vac avail at closed throttle and not at open throttle... but the pedal will not move until the port inside the booster is covered.
>> 
>> the port valve inside the booster will apply the brakes if it gets covered.. and it normally gets covered with the application of the brake pedal... but it can appear to be covered by the booster being full of fluid.
>> 
>> the booster can be a bugger to drain in place..
>> 
>> 
>> ----- Original Message ----- 
>>> Jamie -
>>> 
>>> I believe Bob indicated in his original post that he had replaced the
>>> master cylinder.  Sounds like 1-way check valve in the engine compartment
>>> to me, at least that's been my experience.
>>>> On Sun, Aug 30, 2015 at 9:31 AM, Jamie <jrivers at globalserve.net> wrote:
>>>> <SNIP>
>>>> old master cyl can leak out the seal that is between the master and the
>>>> booster,, and you will have a fluid leak that you can't see..
>>>> Jamie
>