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Tech Question

Wayne Akister, Strathmore, AB, Canada, 1970 Dodge Charger 440

Hi Rick, I’ve been reading and enjoying your tech forseveral years. While I thought myself somewhat capable, I’ve come across aproblem that has me baffled.

> 1970 Charger, blown 440,put together two winters ago. Was a drum/drum PBcar, converted to disc/drum. During resti, the following were replaced withnew/rebuilt. Master cyl, booster, all lines, calipers, pedal assembly,proportioning valve, rotors, pads, flex lines, etc. Also added a Wilwoodprop valve after the stock prop valve.

> Car worked awesome for two weeks, then suddenly seemed that the boostercrapped out, (had to two foot the brake pedal to get it stopped). Replacedthe booster, lasted two days, did the same thing.

> Parked it for the season, and during the winter, replaced the 8.75 with anew Strange Dana, with rear discs. At the same time, replaced the boosterand master cylinder with a MP brakes set up, with the proper combinationvalve (set up for a 4 wheel disc). Re-plumbed the brake lines properly,vaccuum bled the system from the farthest point first, made sure the masterwas properly bench bled, etc, and I still hardly have enough brakes to stopthe car from a crawl. The lines are hooked up properly, everything I canthink of is new or has been checked out thoroughly.

> The only anomaly I have found is that the instructions from MP’scombination valve indicate that the inlet from the master for the frontbrakes is supposed to be 3/16″, and the rear 1/4″. I have 1/4 supplyingboth. Is this a problem? It doesn’t explain why the car braked properlyfor the first few weeks.

> Also, today I changed the master cyl to a different (rebuilt) unit, cardoes the same thing. Tried a vacuum pump hooked to the booster instead ofusing manifold vacuum, same deal.

> What gives? Am I overlooking something basic? The car has been sittingmost of the season already, as I won’t drive it until the problem has beendetermined for sure.

> My fingers are sore from scratching my head on this one.

> Any help on this would be appreciated, as the car is just begging to getout of the shop, and on the street where it belongs……..

> Thanks

Wayne-

Yes, you’re overlooking the fact that stock-type power brake boosters takepositive presure and will blow quickly. The check valve is supposed toprevent this but the stock ones also were never designed for boost.

Either lose the booster (my recommendation, unless you’re a 100-lb weakling)or try a check valve setup from an ’80s turbo Mopar.

You owe me a 6 of Labatt’s!

Rick

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