Strange pressure problem

[Kasimir]

Hi experts!

I have a strange pressure problem:
I have a gradient on a RP C18 column with eluent A: 50-50 (MeOH-H2O) + 5mM Ammoniumformate; and eluent B: 95-5 (MeOH-H2O) + 5mM Ammoniumformate. Analytes are pesticides.
First comes eluent A than B and than A again. During the equilibration with A the pressure increases suddenly! Of course, the 50-50 mixture has a higher viscosity, but the increase is so enormous!!! More than 2000 Psi so that the fittings begin to leak. The column is new!
I had this problem (using this method over months) never before.
Who has an idea?

Thanks!

[Uwe Neue]

The pressure is expected to increase roughly 2-fold between the two solvent compositions at the end and the beginning of the gradient. If this is what you see, things are normal.

It may not have bothered you initially, but the resistence of the column may have increased over time due to contamination.