Dic: let somebody down, let down

[1] let somebody down

• Don't worry, Xiao, I won't let you down.     
• When such advisers fail in their duty, they let down the whole system.     
• Many believe it was his shyness and insecurity which let him down.    
• Sadly, the film is let down by an excessively simple plot. 
• I'm sorry I let you down. Something came up, and I couldn't meet you. 
• I don't want to let you down, but I can't support you in the election. 
• Please don't let me down. I am depending on you. 
• I let down the entire cast of the play.

1. PHRASAL VERB | If you let someone down, you disappoint them, by not doing something that you have said you will do or that they expected you to do.

• CF. v-link ADJ |  The company now has a large number of workers who feel badly let down.

2. PHRASAL VERB | If something lets you down, it is the reason you are not as successful as you could have been.


[2] let down

• Now is no time to let down. Keep on your guard. 
• After the contest was over, Jane let down a bit so she could relax.

to relax one's efforts or vigilance.


[3] let something down

• I let the tyres down on his car.    
• Remove wheelnuts, let down tyre, put on spare.

PHRASAL VERB | If you let down something such as a tyre, you allow air to escape from it. [mainly BRIT] (= deflate)  

..... Cobuild, http://idioms.thefreedictionary.com/