Packard Forum Index » V-8 (1955-56)

I'm not worried about stopping power. I was attempting to determine if the hydro boost was used on small engines (e.g. 4 cyl) due to the inherent low vaccuum VOLUMN of small engines (and diesels yes).

If used on small cars then very likely suitable for a BTV transplaant and thus retaining the stock oem Packard pedal.

However that depends on the stroke and diamtere of the GM MC.

One other thing to consider is the rather steep incline that the GM Hydro boost would have to be mounted on. i.e. simply replacng the BTV in the original BTV possition. Importaant for those with factory AC.

Maaybe the GM hydro boost has to be level???? Or as i have seen on many GM cars with the old vacuum assist (e.g. 89 Caprice) the MC and power unit tends to set rather high at the front and lowww nearer the pedal. DIRECT OPPOSITE of the oem Packard BTV.

Quote: If the modern GM hydo boost has the same size MC piston as the OEM Packard BTV then it would work just fine with the oem Packard pedal (set up as oem 1:1 pedal ratio). The next question would be how much STROKE does the GM have compared to the Packard BTV.

Not sure I agree with you on this

If you go to Master power brakes web pg or piratejack.net you will see the effects of pedal ratios. Most power systems are 4:1 or 5:1 ratios and manual brake systems have a pedal ratio of 6:1 or slightly more. They also show pedal designs ie hanging or under floor and how to calculate ratios.

I believe the hydro unit may be able to be mounted to an angled toe board. I say this because i remember it in several vehicles. IIRC tight clearance on the astro fire wall. fairly vert there and quite an angle on the pass cars as stated above. just the master resevior difference?

Make no mistake. this is no minor undertaking. The pressure hoses, the pump press n vol, the pedal ratio and bracket. I am just adding any info as help to those making the decision to retrofit. I am not championing this avenue. Does the 55 dash un bolt? can it be removed? Why not a 90% under dash power unit. Why is it so important to keep the factory pedal? For aaca judging? I really would like someone to answer this for me. I have pondered this before. Is keeping the exact pedal set up a must have in this improvement?

Let me put it this way:
I know for a fact that a 13/16 piston mounted 1:1 in my 56 Exec is more like a manual brake. In fact NOTHING like a power brake whaatsoever.
I used an 86'ish Honda Civic power brake and mcc unit mounted directly in the samee location and pedal connexion as the oem BTV and used the oem Packard pedal. Just like a manual brake at best.

So 13/16 piston is too large. To top that off the honda mc ran out of stroke especialy upon minor brakeshoe wear. Remeber, we do NOT have suto adjusting brakes shoes. The BTV has a rather longstroke compared to any other mc.

I am trying to remember in the trucks with the hydro brakes and power steering if the high press hose went from pump to brake booster then to the steering box then back to pump res or if it fed st box first then brakes. Simple visual just follow the hose. The press side off the pump has a threaded fitting and low press return just a rubber hose and clamp

It is plummed inline with the steering and the claim on reserve braking is as follows.:
Back-Up
Similar to the vacuum booster, the hydro-boost is equipped with a backup or reserve in case the source of pressurized fluid is lost. A failure in the power steering system, such as a broken hose, broken power steering pump drive belt, or failed pump, would result in a loss of pressure to both the hydro-boost and steering gear. The hydro-boost uses a high-pressure accumulator to store power steering fluid under pressure in the event of a failure. There are two types of accumulators used, some hydro-boost units use an external accumulator, while others incorporate the accumulator in the power piston. The accumulator could be either of the spring-loaded variety or nitrogen-gas type.

In the event of a loss of pressurized fluid, the accumulator will provide two to three power assisted stops. Upon the first application of the brakes after an engine stall or loss of power steering, you would find approximately 60 to 75% of the normal assist available. If you were to release and apply the brakes again, you would find approximately 30% to 40% assist, then again approximately 10% to 20%, until you have depleted all stored reserve assist. Once you have depleted all of the stored pressure, the brakes will no longer have power assist and will be manual in their operations

In reply to your post 37: Yes there are no mysteries here. You are using the 13/16 bore. You are trying to use a sm bore mc to get your pressure up. The small bore moves less volume. You will not be moving the fluid you need to. If you have a mc with a 1 in bore, and an appropriate booster be it vac or hydro and a pedal ratio more like 5:1 you will have your force multiplication and your power assist.
by the way I saw the other thread on the retrofit. "Reinforce the firewall with plates inside and out" . There has to be a better way!