Забыл сообщить общественности, что пару недель назад заказал на eBay нормальный клапан, специально заточенный под пневматические гудки под 12 вольт.
Вот такой:
Вместе со страховкой и доставкой вышло $57.
Вот описание продавца:
Brand new in the original package! Used with air powered horns that require an on board air system for 12 volt vehicles. When the vehicle's horn button is depressed, the Model 804-EV becomes electrically energized, opening the solenoid and permitting air to reach the horn. The solenoid remains engaged as long as the horn button is depressed. Max 150 PSI. The valve easily installs to any 1/4" air hose with its brass fittings and comes complete with all needed hardware. Spanish instructions included.
Specifications
* Hose fittings: 1/4 inch
* Size: L 3 1/2 in. x W 3 in. x H 2 3/4 in.
* Voltage: 12 volts
* Solenoid current: 300 mills