  Individual mode frequencies have been detected in thousands of individual
solar-like oscillators on the red giant branch (RGB). Fitting stellar models to
these mode frequencies, however, is more difficult than in main-sequence stars.
This is partly because of the uncertain magnitude of the surface effect: the
systematic difference between observed and modelled frequencies caused by poor
modelling of the near-surface layers. We aim to study the magnitude of the
surface effect in RGB stars. Surface effect corrections used for main-sequence
targets are potentially large enough to put the non-radial mixed modes in RGB
stars out of order, which is unphysical. Unless this can be circumvented,
model-fitting of evolved RGB stars is restricted to the radial modes, which
reduces the number of available modes. Here, we present a method to suppress
gravity modes (g-modes) in the cores of our stellar models, so that they have
only pure pressure modes (p-modes). We show that the method gives unbiased
results and apply it to three RGB solar-like oscillators in double-lined
eclipsing binaries: KIC 8410637, KIC 9540226 and KIC 5640750. In all three
stars, the surface effect decreases the model frequencies consistently by about
0.1--0.3 $\mu$Hz at the frequency of maximum oscillation power
$\nu_\mathrm{max}$, which agrees with existing predictions from
three-dimensional radiation hydrodynamics simulations. Though our method in
essence discards information about the stellar cores, it provides a useful step
forward in understanding the surface effect in RGB stars.
